I am 21.I want to be fit.actually I am looking like a 15 years boy.I want to be manly.I never tried to be fit like going to gym like that.I want to be a 21 y man not 15 y boy.I need some tips to become fit.please send some tips to me.

Well for one, becoming fat is nowhere near what you want. Becoming fat and starting to look fit are not the same.

I'm going to leave with a couple of questions about yourself that I'll look into later on and a few links you should check out.

Questions are as follows:
- How does your diet look like? Do you have a good balance of fiber, hidrates, water, fat and protein?
-How often do you work out? (No, you don't need a gym to work out)

Anyway, I want you to check these:

Beginner's Health and Fitness Guide

Will give you a large overview of fitness in general.

If you aren't working out yet, try this:

YouTube - Work Out with ice1cube AGAIN!

I'll come back to check on this later.

First of all - your headline reads "how to become fat", not fit :) - two very different things. Check out this link - I think this may help: http://www.gain-weight-muscle-fast.com/

And - well. If you want to be fit you DO have to start working out.