I just spent the last hour writing this reply. You better read it *menacing glare*
Firstly, always err on the side caution with the weights you use. There's a program out there called StrongLifts. It's not perfect, but you could do a lot worse than it as a beginner. The program layout is simple:
Day 1: Squats 5x5, Bench Press 5x5, Barbell Row 5x5
Day 2: Squats 5x5, Overhead Press 5x5, Deadlift 1x5
You train 3 non-consecutive days per week, and alternate between training days every time you train, so over 2 weeks, the schedule looks like this: Day 1, off, Day 2, off, Day 1, off, off, Day 2, off, Day 1, off, Day 2, off, off (off-days are days in which you don't train; typically these are Tuesday, Thursday, Saturday and Sunday).
For squats and presses, you start with lifting only the Olympic barbell (45lb) for your 5 sets of 5 reps. For rows and deadlifts, you're supposed to start with some extra weight according to the book, but I'd only start deadlifts with added weight. For deadlifts, you add 10lb to the bar every time you train. So, say your first session has you lifting 95lb total, next deadlift session you lift 105lb, then 115lb, then 125lb, etc. For everything else, just add 5lb to the bar every time you train. Your squats and deadlifts will progress at about the same rate doing this - even though deadlifts go up in bigger jumps, squats are performed twice as often, so each of the two big lifts go up by 30lb/fortnight.
These are pretty safe weights to start with, although the overhead press might be too heavy, depending on your intial strength. If that's the case, forget about going by the book, and just start with some light dumbbells, say 10lb in each hand, and work your way up until you're doing 25lb/hand, then switch over to the empty barbell. For everything else, there shouldn't be much hassel, so you'll be able to just focus on getting technique right and doing your prescribed reps, then going home, and doing it all again 2 days later with a little more weight on the bar. Using this template, assuming you don't run into any trouble on the program, you'll be squatting 225lb, benching 135lb and deadlifting 275lb after 12 weeks, and you'll be doing it safely.
By all means, this isn't the only program you could do, or even necessarily the best, but it's a good program for a beginner, and that's all that matters. It focuses on the things you need as a beginner: linear progression, learning a few important exercises and becoming competent at them. Any program that achieves this is a good program for a beginner to be doing.
On Technique
Technique-wise, the most important things for almost every lift you'll ever do is keeping your back in a strongly supported neutral position. This means extension (but not hyperextension) of the upper and lower back vertebrae. Standing upright, you'll achieve this simply by finding the vertebra that's at the same height as the bottom of your chest/top of your abs, and pulling everything in towards that point. This will cause you to lift your chest, pull your shoulders back and down, and contract all the muscles of your back.
Under load, you'll also want to reinforce this position with your abs. Some geniuses out there have managed to trick the world into thinking that drawing your belly in towards your spine does this, but doing so requires a shallow breath or no breath at all. What you want to do is take a deep, deep breath down into your belly, so that your abdominal region expands as you breathe in. Then hold that breath down there and tighten your stomach. This increases the pressure inside your chest and your abdominal cavity, so when the abs contract at the end, they put a great big force against the spine form the front, providing a lot of support to the spine. When lifting weights, take that deep breath in and contract your abs against it, holding the breath down, before you start each rep, and hold that deep breath until you finish the rep. Then, when you're in a safe position (lockout/the top position for squats and bench press, or the bottom position for overhead press, rows and deadlifts), exhale and take another deep breath before continuing on to the next rep.
In all lifts, place your weight over your heels. With squats in particular, it usually helps to place the weight over the outside of the heels, as people often allow their ankles to cave in while squatting. With squats, overhead presses and deadlifts, the barbell should be over the midline of the foot from side view at all times. This is the most efficient place for the bar to be -- putting it further forward or further back will do anything between making it harder to lift the weight and actually making it more dangerous. Fortunately, you won't have to worry about this much while squatting -- just sit back and keep your chest up. With deadlifts, just start with the bar over the top of your shoelaces, bring your knees forward just enough for your shins to touch the bar, and then keep the bar against your legs throughout the lift. With the overhead press, this becomes a little more tricky, and it means you'll have to lean back slightly at the start of the lift, then bring your chest forward once the bar passes your forehead so that you can get under the bar.
For the bench press, you're lying down, so the only thing you need to keep the bar over is your elbows. For rows, try and start with the bar over your shoelaces, but keep your knees back and don't try to touch the bar with your legs as you lift it.
Every squat performed starts and finishes in a standing position, with hips and knees locked out. The bottom position of the squat places your hips just below your knees, and getting here safely normally requires a stance of shoulder-width or wider between the heels, and toes pointed out 30-45 degrees, while driving the knees out to the same angle as you go down. Often people's bums will tuck under towards the bottom of the squat, which means that the lower back has gone out of extension and into flexion -- not a position you want it to be in. There may be some flexibility issues at play here, but most of the time it's a control issue. Focusing on contracting your glutes at the start of every rep, pushing your hips back as far as they'll go as you get towards the bottom of the lift, and driving up with the glutes on the way back up will usually remedy the problem.
There are often similar (but more pronounced) problems with rounding of the lower back with deadlifts, rows, and any other exercises that involve being in a bent over position. 99% of the time, this is because people think that bending over is a movement of the lower back. In order to keep the lower back in proper, rigid extension for lifting, it helps most people to think of unlocking the knees, pushing the hips back and pushing the chest forward, or pivotting around the hips, to get into a good bent over lifting position.
Deadlifts and rows both start and finish with the bar resting on the floor, elevated 8 inches/20cm (the height it will be with a standard 20kg/45lb plate on either side of the bar). If you aren't using the big plates, and don't have lighter training weights with the same diameter, pile up some blocks or weights underneath the bar, or set up safety bars in the power rack to get the bar elevated to the correct height.
To perform the deadlift, from the start position, wrap your fingers around the barbell with a grip close enough that your arms are right outside your thighs. Push your heels into the floor as if trying to push the floor away from you, and drive your hips forward until your hips and knees reach lockout. At this point, you should be standing upright, with the bar across your thighs. Unlock your knees, pivot around your hips and lower the bar to the floor again. Your arms should stay straight during the entire lift.
To perform the row, from the start position, keep your knees back, pivot around the hips, and wrap your fingers around the bar with your hands a little wider than shoulder-width apart. Think of bending the bar as you lift it, pulling your elbows in towards your side and driving your elbows up to the roof until the bar hits your lower chest/upper abdomen. Lower the bar to the floor to prepare for the next rep.
Each rep of the bench press starts and finishes with your elbows locked out and the bar over the chest. Like with the row, think of bending the bar or pulling your elbows in towards your side, and lower the bar down, keeping it over your elbows and keeping your wrists straight. You'll most likely end up with the bar right at the bottom of your chest. When the bar touches your chest, drive the weight back up to lockout.
It's worth noting that elevating the chest and squeezing the shoulder blades together, along with the above technique of pulling the eblows in towards the sides of the body will generally make the exercise safer for your shoulders. Do not attempt to flatten your back against the bench. Much like how drawing your belly in does not support your back, flattening your back against the bench also leaves your spine prone to injury, while actively tightening the back muscles (and in doing so arching the back off the bench) gives support to the spine. Just make sure your glutes and shoulders are on the bench at all times throughout the lift, and that your heels are planted into the floor.
Each rep of the overhead press begins and ends with the bar resting across the shoulders. Keeping the knees locked out, press the weight up very close to the face, close enough that you could kiss it. Don't kiss is, it has girl germs on it. As the bar passes overhead, get under it, shrug up your shoulders and lockout your elbows.
