Starting out for any given weight, do the weight that you can get good form with on each rep. If your muscles can handle 100kg for 10 reps, but you only do 50kg for 5 reps, that's fine when you're learning to lift an external load proficiently. Better safe than sorry, and until you're particularly skilled and strong at a lift, your technique will degenerate long before you feel like you can't do any more.
I like the 5 rep range for beginners learning the movement pattern under load, because the more reps you do, the less accurate your technique becomes. Early on, you want to practice technique more than you want to fatigue yourself. It takes 500 good reps to learn an exercise from scratch -- it takes 5,000 good reps to correctly relearn an exercise once you've developed bad motor patterns and need to fix it.
Do 2-5 sets of 5 reps with a weight you can easily handle for every set and focus on good technique on every rep. If you get every rep with passable form, increase the weight next time. Over several weeks, the weight will eventually pile up and you'll really more on your muscualr strength to lift the weight. By this time you'll have good motor pathways laid down, so you'll be better equipped to handle the heavier weights once you reach them.
After 1-2 months of training, you should reach the point where you're coming close to failure frequently, but you should avoid actually reaching failure on each exercise. Say you're doing 3x5 for an exercise and you've got good technique, you should be getting towards the point where on the first set you could have done maybe 6-8 reps but you stop at 5, on the second set you might have been able to do 6 reps, but again you stop at 5, and on the last set you can do your 5 reps, but might have failed had you gone for a 6th rep. Again, this is about the intensity you want to be training at after a couple months of building up a base of good technique so that you can actually handle that intensity without your form crumbling. Early on, light and easy is fine (and recommended) so long as you emphasise getting your technique down.
Once you've got solid technique, other than trying to push heavier and heavier weights for the same number of reps, that's also a good time to back off the weight a bit and increase the rep range, if you have a good reason to do so. If you want to increase the number of reps you do per set, make it a gradual change. It'll be easier to go from 3x5 to 3x8 than to go from 3x5 to 3x12, even if you drastically reduce the weight to accommodate the extra reps. The reason for this is that learning good technique is one thing, learning to maintain good technique for an extended period is another.
BTW. Conventional wisdom is that 1-5 reps = strength; 6-15 reps = hypertrophy; and 16+ reps = endurance, or something to that effect. While there's some truth to this, in reality, you'll get stronger no matter what rep range you're in (if your 20-rep-max goes up by 10kg, then you've gotten stronger for 20's). And building muscle is a function of training + diet + hormones + genetics. For a beginner this has two implications: 1) As they don't say often enough, you aren't what you don't eat. 2) Simply by getting stronger in ANY rep range and eating right, you will build muscle early on. I haven't done "hypertrophy" training in over 2 years, and yet I've grown more in the last 2 years than I had in 4 years prior to that.
If you're desiring to take advantage of the ease with which you claim to put on muscle, then (once you've got good technique) lift heavy and eat big. If you don't want to put on much muscle, then lift heavy and don't eat big.
I'm babbling.
I'll shut up now
