To the OP..
You should have posted this question in a fitness forum or something like bodybuilding.com forums. I have seen terrible advice so far in this thread. Who was that poster with the "Why stop running? That's stupid" Lol. If you want to get big, you can't run.
And : Muscle does NOT convert into flab.. nor does fat convert into muscle. Like some poster said that guys get big.. then they don't work out.. and all that muscle converts into fat.. haha.
Genetics play a big part obviously.. but don't be discouraged by that factor. I was 6 ft tall and 120 lbs when I was 24 (yeah, I used to go to gym) I tried all sorts of weight gainers, protein shakes, etc.. nothing worked. I was almost resigned to the fact that maybe it's bad genes (my parents are Indian.. so they're 5'4.. and very very skinny)
Everything changed when I started lifting heavier, and doubled my calorie intake and ate clean. It's easier said than done, but try eating 6,000 calories in one day - and all of it being clean food!
Oatmeal, eggs (yes, the yolks as well - and please, not raw - raw eggs are not assimilated by the intestines) chicken, meat. Avoid fish if you can while you are bulking up.. fish is great when you're on a cutting diet. Make sure your protein intake is at least 200 gms a day.. whole foods preferably but a protein drink is ok. Complex carbs are your friend obviously, and so are good fats (fish oils, flaxseed oil, almonds, etc)
If you don't eat and don't lift heavy.. you won't grow. You will put on a bit of fat, but that is VERY easy to get rid of. I got rid of 9 pounds of excess fat (post-holiday meals!) in 7 days by just changing my diet. The body responds in an amazing manner if you know how to tune it.
Please avoid junk food - it's easy to get 6,000 calories by eating McD's .. but I really don't recommend that at all. When I talk about lifting heavy I'll give you an example..
Most people you see at the gym, lift the same weight - it's a routine. For e.g. if you're doing bicep curls with a barbell.. you might start with a 15 lb plate on each side, then go 20 lbs.. then 25.. people do 3 sets, and that's it for them.
You *have* to go heavier than usual if you want to grow. If your last set is 25 lbs plates on each side.. and you can do maybe 4-5 reps.. do another set with 30 lbs on each side. Doesn't matter if you can only do 2. That's fine. Do the same thing next time. Your muscles will adapt, and pretty soon you will be doing 6-8 reps with the weight.
Then I could get into power training and stuff.. but that's not really required here. Don't lift too heavy either that you injure a tendon or something. Just put 5 lbs more on your max weight.
Last thing to get big - squats. Do squats the right way i.e. deep.. till your thighs are parallel to the floor. Trust me, try squats.. and see what a difference it makes. Good luck!
Edit - Ahh, don't forget the rest factor. When you lift heavy you stress your nervous system pretty good.. so sleep a good 8 hours or so every day.