LovetoPump, u have a good base for a 15 year old, how much do u weigh and how tall are you?.
You are already really lean with and awsome bicepe peak that alot of guys would die for!. what i suggest is start taking protein supplements 2 times a day about 25-50 grams of protein per serving. start eating alot. try and stay away from fast foods to often. and only work out 4- 5 days a week max

Day
1--- chest ( could add in triceps if u dont do tris the next day)
2-- shoulders and triceps
3-- Back and biceps
4--legs and calves
5--any thing u feel u neglected that week a bit

remember the key is lifting heavy but with good form if u want size, stay away from doing 12 reps unless its your warm up set. stick to 6-8 reps and even 4-6 reps on really heavy days.

dont touch creatine untill u have your diet down pat and your training down, try to eat six small meals a day high protein moderate carbs