I read an interesting article on Arnold Schwarzenegger's summarised weight training
here.
Most of it is common sense and although an interesting article worth a read, I picked up that all you really need to know is this:
Perform a minimum of four sets of whatever exercise your doing with 8-12 reps per set. Apparently this is a sufficient enough workout to stimulate muscle growth and all that.
If you can't figure out how much weight to use, just be sure you can barely do the last rep of the last set and this should do the trick. Eventually you'll get used to this the more you do it, so in order to progress, add more weight over time.
Obviously don't work out the same muscle 2 days in succession or you'll be overtraining. Oh, and eat well and stay off the "juice".
Basic, but it works.