I want to develop lean muscle, i don't want to get big. Should i do low weight and high reps, i have heard that is the best way, but i have been hearing more and more about group muscle traning not individual muscle yraining, which is better?

IMO, the best exercises for muscle are squats, deadlifts, bench press, shoulder press, bent over rows, chins and dips. also throw in some calf and ab work. some direct arm work is fine just dont spend alot of time on these. stick with simple splits like push/pull and upper body/lower body.

to develop your muscles you need to work them. low weight and high reps is good for beginners, when learning proper form, when learning a new exercise and for endurance. after this, 1-3 sets of 8-12 reps will suffice.

getting big takes time and you'll reach the point where you want to be long before that happens.