Depends what you want to do. Generally the rep schemes fall like this: 1-5 = Strength 6-12 = Strength and size (more size than strength) 13-20 = Endurance fibers working (definition, yet muscle adapts by shrinking to last longer)
If you're eating a diet to lose weight, then it won't matter whether you do 1 rep sets or 20 rep sets because your diet isn't set up to build muscle. In order to build muscle and gain mass(combination of muscle and fat), you ave to be in a caloric surplus over your maintenance level.
Eating more calories than maintenance = gain mass Not eating more than maintenance = no gain in mass
If somehow you DID gain muscle from being on a diet and training routine made for losing weight, you'd be one of the few (and I mean 2-3% of population) with some godly genetics.
As a 15 year old female I assume you are going for muscle tone verses muscle mass. As such you should aim for low weight high reps. Start with just the 45 lb. bar with 3 sets of 10-15 reps of curls with the bar 20 sit ups without weight and 20 sets of squats with the bar and 5-10 situps. Do this work out 3-5 times a week. As you progress you can start adding weight, reps, and sets in small increments.
When you are still young and growing you should not do to much weight training because it can interfere with your growth and joint forming. it is best to do weight-bearing exercises which means only using your bodies own weight. 10-12 reps in a set, do 3 sets. It does depend on the exercise?
Light weight weight and a lot of reps to tone, Heavier weight and fewer reps if you want to build mussel mass.
Ask someone like your PE teacher, or a physical trainer who has seen your body in person and knows what your goals are.
i cant even lift the bar thing for reps!! >.<
i wanna get the mass (sadly)
I do 3 sets of 10 reps.
awesome, thanks man