Should i combine low/heavy and high/light rep weight training in each of my workouts?

I heard that there are benefits of both low/heavy reps and high/light reps. So should i do both in a workout? eg. Do heavy weights low repetitions and then light weight high repetitions afterwards? Or should i just do heavy/low reps for 2 weeks and then switch over to light/high reps for the next 2 weeks. By the way i'm 15 and iv been lifting for about 3 years. I just want to get stronger and make my muscles big and toned (hard and shaped). I lift about 13 kg for heavy and about 8kg for light.