THE GYM! Simple as that. I am just 16 and was 2 years ago quite skinny, so i just started eating and going to the gym. Now i got pretty nice chest and a 4 pack. Going for a 6 pack before the summer.
Even so, this is what you need to do.
Eat alot (I am 1,90m and my weight is 80-90 kilo). When you just started go eat in the morning and try not to eat ALOT in the afternoon, if just starting with the gym go 2 times a week. After you body got used to lifting start going 3 times than 4 times. NOW, it takes alot of motivation to go there. What keeps me going is my GF, i always go with her, she most times goes and starts running and i am lifting. And like every man i wanna impress her, and that is what keeps me on going.
If going this is how you should start:
First running, just to warm up a bit. (I go jogging for 10 mins, thats quite alot of time and i am always sweating but 5 mins is also good)
Than train your shoulders and chest, (10 min each and try to take a bit heavy weights)
Than train your back, 10-20 min.
Than you can choise arms or belly, i normaly go with arms (Bisepc and trisepc). But like i said, i want a 6 pack
its totaly up to you. both do 20 mins for.
If doing this right you can stay there for like 1-2 hours. If i really feel good i can stay training for 4 hours. I do not recommend to train for 4 hours
Sorry for my bad english ._.