RE: Warm up sets?
Warm up sets are nto high rep.. just do the same amount of reps your going to do for the exercise.
keep the weight light. The warm up prepares your muscle for the exercise, and will make the muscle start producing energy etc. Also the warm up helps with injury prevention, since it helps to stretch ur muscles.
In fact, u will find that u can do more weight for ur exercise if u warm up before.