Many gym-goers lifting the same weights week after week — this is just one way to keep your body from changing. So how do you figure out how much to lift if you don't know your 1-rep max?
if you're a beginner:
1) Choose a weight you can only lift with good form for 12 to 16 reps (which constitutes 1 set). You don't need to go into complete muscle failure, but make sure you're challenging your body.
2) Begin with 1 set of each exercise, slowly working your way up to 2 to 3 sets (adding a set each week).
3) When you've added sets and have a solid foundation (after 6 to 8 weeks), add enough weight so that you can ONLY do 8 to 12 reps.
4) Continue to progress by adding a rep each week until you reach the max reps (no more than 16); then increase your weight again and drop your reps back down to 8 to 12.