For a newbie, how do I calculate the amount of weight I should be lifting each set when I'm first starting out? Many books and articles give workout plan suggestion that contain exercise, rep, set, and technique information, but none, that I know of, explain how to figure out the starting point for weights.

From what I've been reading, to gain muscle you have to overload them with weight. Most trainers suggest 3 or so sets of 4-6 reps. If the weight is too light, I won't gain, and if it's too heavy, I'll risk injury and proper form. I was just curious to know if there was a system/technique/best practice for beginners to be able to quickly gauge how much weight I should be lifting for a particular exercise.