Beast, it all depends on your needs/ideals for the extra muscle and what you'll do with it.
Are you sporty? If so, what sport/s? I see football is already one.
Is it to make you feel better, more comfortable, aesthetics, even build up to bulking for weights lifting or body building?
Montanaro's first point is bang on.
If you can't fuel the growth, you're gonna have issues.
Protein is your biggest friend for building muscle. Calories helpenergise your exercises. Aim not to be short on either, and don't get too excessive with either as it'll just be waste or turned to excess fat.
Jake is smack on as well.
To build muscle you're actually micro tearign muscle.
Resting is very improtant regardless of age.
Resting is when the body repairs itsself to the newer levels you're asking of it, and no, just nightly sleep doesn't count as
resting being ticked off.
When starting your training, be aware of the muscle groups you'll be working and let them rest the following day. Legs one day, upper body the next, LIGHT aerobic exercise... and so on. Have a complete rest day the day after your heaviest loading (most likely your sunday football)
Being 15, your best bet is actually playing and training often.
High Performance kids training here in Aus plan out that it's best to train and play often (4 trainings, 2 games per week) until adulthood, push explosive and agility based weight training until 27, then maintain levels through to cessation of elite sporting carreer. And we produce kids across all ranges of sports, it's the following education we struggle on, hence in football those who head overseas end up better and in the Socceroos.
If you can find another training sesion (or two) to get in on a social saturday game, do it.
A training could be just you and your mates doing your own sesh for half an hour. Just get the ball at your feet as often as possible.
Build on technique and all the building blocks of football. In three years start hitting up the weights, but recognise the style of fitness and exercise that's suited to your sport and adapt or you can easily end up negating what you need and not enjoy your sport as much.
Read. Self educate.
Don't take any singular write up as gold, validate it. Find more literature that agrees.
An essay, yes, BUT if your exercise is tailored towards what you already do in sport, and if you teach yourself with an open mind to options then you'll have a much easier time maintaining the self discipline to keep doing them.
... If it's not fun, you'll not keep doing it.
So a very rough guide:
Monday - REST Recover from thrashing your body in game
Tuesday - light training (skills) and weight session
Wednesday - Light legwork again, vary it but keep it under an hour. Skills training one week, jog the next. Just leave the upper body alone.
Thursday - Hard training
Friday - moderate upper body weights session keeping legs fresh for the weekend and don't overwork your core. Alternate this day every 2 weeks with resting.
Saturday - Social game and a light jog/swim
Sunday - Full bore game. Leave everything you have on the pitch.
NEVER FORGET TO STRETCH, WARM UP AND WARM DOWN!
Even before doing weights, stretch the areas you're going to work.
