I'm 14 as well and I want to start getting some muscle.
I've decided to use my school gym once or twice a week for weights.
People say it stunts your growth and basically if you use the simpler weights then it doesnt.
Do some of the bicep ones and it should be fine.
The rest I do in pushups.
The pushups work on the chest and bicep strength in particular. But it's quite a good all round workout.
It does abs a bit if you keep them tight which you should. triceps almost like your biceps and even a bit of leg strength. This is how you get those massive muscles in your chest.

Do some crunches(half sit-ups) on most nights just to get your abs working.