Thanks for that, you'll be suprised how few are able to give so much information. The detail means your age really isn't an issue, which is good news for you obviously. Some things I can't advise to younger (bones still not fused) people but this should be OK.
Technique. Teaching someone to walk or run properly is very difficult, most of us assume that we would do this automatically and many of us are wrong. I was a good level runner and still had to correct my arms to get the best pace and safest style.
Walking uses all of the body as most know, running is the same. Despite this being common knowledge people ignore this when looking at technique. When correcting technique bring the pace down a bit, even if it makes you slower than some walk, it is important enough to delay for. The below starts at the bottom and works up, and for a while you will find yourself getting some things right and others wrong, don't stress, you will get there.
Strike pattern. Different at different paces, you are into football so will need to be able to jog around and sprint so have to learn both.
Jog strike is slightly on the outer heal with foot slightly in front of or below hips (the safest is below, but if wanting to extend strides you will strike foward), you then go along the outer side of the foot across the ball and take off with your big toe. When you look at the shape of a foot this starts to make sense, and most people do this naturally, if you don't it needs to be corrected as everything in your body is set up for this and if this is wrong you will cause damage everywhere.
Sprint strike is simpler because it doesn't involve your heal at all, it is a matter of using the ball of your foot and driving forward through this and your toes. Low arch find this difficult because they have very little give in the tendons below the feet so the spring required can be painful, I'll cover that later. The most important thing in sprint is not to land whole footed. If a spint becomes perlonged you move from ball to heal not flat.
There are many worrying about striking with the heal, and a lot of advise against it. If your toes face skyward when you land, that is an issue but if they are a few cm above the ground you are spot on.
Legs. These are doing most of the noticable work so are obviously very important especially the knees. My most loathed expression from idiots about running 'get those knees up.' Running is high impact and due to it's duration if done wrong damaging impact, the closer your feet stay to the ground the less of an issue this is, landing from 5cm above the ground is not going to send anywher near the shock of 10cm etc. The knees bend is to bring your feet forward not up, the whole idea of running is to propel you forward so as you will notice a lot of this advice is about moving in a linear fashion forward and back. Your legs will be close together throughout the run, over distance you will often find people cross over their legs slightly meaning the foot prints would be virtually in line rather than two, sprinting there are two distinct lines. As long as you land with your foot on the same side as the leg is on your body this means you are not swinging your hips too far to cross over and overstressing your legs or risking your knees going out of alignment. Considering how important the legs are that is it, legs go forward and back, not up, don't corss over either go in one line or two keeping compact.
Torso. Easy to miss any issues here as most think of it as purely for breathing when running. Posture is as important on a run as any other time, and is easy to get wrong. The sargeant major style shoulders back, chest out, stomach in, is an incredibly good place to start, you won't do this fully while running because you need to be relaxed enough to breathe deeply, but aiming to stay close will work. If you aim for a straight back that will give you problems because the spine isn't straight, good posture at the torso will also avoid some of the duck shaped running I sometimes see when people have their backside sticking out. When jogging or running distance breathing is important and you will need to breathe all of the way down to what is called the pit of your stomach, but keep it relaxed, if you are having to force the breathing and want to go a good distance it won't happen. Sprinting is a different deal as you likely won't be breathing but will instead need to lean your torso forward while still keeping the posture correct, the drive from your legs will need you to counter balance and the torso is where this needs to happen.
Additional torso detail for females. You didn't specify so this is here. Support is key, A cup breasts to triple HHH, they have to be supported correctly to ensure you maintain proper posture and don't damage your back. If you aren't wearing something that does this well you will become one of the many who uses hands or arms to do so while running and messes up everything else by doing so.
Arms and shoulders. Often neglected and the source of the worst techniques around, including what used to be mine. Getting these right is insanely simple, but if you aren't used to it also incredibly un-natural feeling at first. The arms need to go the way you are running, they are your main steering. So if they are going across your body it is the equivilant to steering one way then the other in your car to go along a straight road, the way that would feel in a car is what you do to your body. If going straight which most running is, your arms go forward and back, hands, elbows and shoulders in parellel all of the time, a bit of a turn one arm moves less, and that is enough, sharp turn you start to direct. Shoulders need to be relaxed, my run with major tension in their shoulders and wonder why their neck and back hurt at the end, it is because they have been having to support the weight of your arms in an elevated position. Don't pull your shoulders down, but run with them where they naturally sit, not lifted at all.
Final and in some ways the most difficult part if running for distance, stay relaxed at all times. With all of the technique pointers rolling around in your head this will be virtually impossible at first and you will hate me for saying it, and that is as it should be. Nothing I can do will make this part easier, but as the technique comes you will find it easier to relax as you run. Sprints obviously are a more tense affair, but they don't last.
If you find treadmills suit you better and are able to get to one of these regularly that is your path. Personally I hate them and prefer to run outside, but when it is snowing or stormy outside you can laugh at my stupidity for not using the traedmill in the warm and dry.
There are people who just run perfectly without any coaching or correction, most of us hate them with a jealous passion, thats OK too. Truth is though if you see someone running perfectly they have probably had to fix issues and only look that smooth because it was years before. I am over my running weight by almost 50% and can honestly say that despite this I have never injured myself or given myself more than severe aches from running. The only way to make this sort of boast is by doing it right. Correction cost me speed for almost 10 months, but after that I ran my best ever half marathon, so it was worth it. Even now I run at a pace that most think is good, despite my additional mass, so it definately works.
Low archs aka flat feet is not uncommon so there are some things you can do to help this. Obviously shoes are a major thing and you have done that already. If you haven't had a gait analysis it is definately worth doing. The best shoes are hard to find without it. Being lucky I am a neutral runner so it was easy, for you it will be more difficult but also more worthwhile. The difficulty will come deciding which to work on first technique to be sure the shoes suit when running well or shoes to possibly help with technique.
Stretching and strengthening. For you to get the best out of your run you will need to do the part of training I neglect almost totally and know I shouldn't, warm up stretches. You will need to be warmed up slightly before stretching so either some gentle exercises at home or a very steady jog to get the blood flowing slightly faster. Once this is done you need to do some short stretches, 5 - 10 seconds should be enough on each area, with one exception. You have issues with low arches meaning the tendons in your feet are very tight, so these will need more attention than the rest of you, as such you will need to stretch this more to prevent issue, but never more than 5 - 10 seconds at a time, longer will generate unessecary risk of overstretching before tension. To do this make stretching your foot the first stretch, this can be done from a kerb, against a lampost, anywhere you can push the front of your foot against while leaving the heal lower down. Then do a couple of other stretches and come back to the feet, and finish off with your feet again.
Make sure you stretch everything at the end of every run, if it takes a long time, give what it needs. The body should feel relaxed at the end of stretches, not strained or tight. Don't use ballistic stretches, these aren't generally a good idea for most people anyway, and can be the short road to injury if you have tension issues already. Relaxed static stretches with gradual progression are your friend here, sudden bounces will mess you up.
Strengthening. Something to try and feedback on, it will either be great or horrible, if the latter stop immediately do not persevere.
Sprint circuits. This is where you sprint one way, do 10 of an exercise, then back do another, etc. Number of circuits through this is up to you. Be sure you warm and stretch before and cool down and stretch after.
Below are the excercises in order. Most are standard and easy to understand, one is modified to protect your feet, if you feel safe to do standard instead by all means do so.
Squats, depth is up to you, want at least right angles ideally. Heals must stay on the floor.
Press ups, can be box style, straight body, any way you like.
Burpees to toes. Normally these have a jump, but I want you to simply roll up onto your toes instead so a jump without leaving the floor.
Side heal touches. Standing up, then leaning and bending knees slightly to enable you to touch one heal at a time. One side = one rep.
Knee bend lunges. Start with feet in position, one forward the other back, bend both knees so the front one goes forward, back goes down. Five one side five the other.
Bent over rowing. If you have some small dumbells or a bar to do this with great, if not a stick, two bags with something in them of just doing the movement is fine.
This is not a balanced circuit, it is leg heavy, but should be enough to avoid letting other areas become weaknesses.
If it works great, if not let me know.
You have one big advantage on many. You are fixing this at 16 years of age, meaning by the time you are 20 the chances are you will be running with a body in good condition for it with good technique. Most don't bother until later in life and for some there is too much damage to fully fix.
