As ur 15, dont jump straight into gym, just start press ups, chin ups, sit ups, they will help u out properly when u get older and join a gym, try get a partner to join up with u, as motivating each other helps out alot, also recite loads of bani, as going gym can get u off the path.
The major ones if u want strength are:-
meet guru ji atleast once a day
up the amount of bani u recite
get a sarbloh mala and jus do simran, when ur walkin to school, getting the bus, playing COD :D
and touch ur mum and dads feet every day
that will give u alot of strength with out going gym
