Go to the gym or have a lot of sex
gym is a big one, or just more exercise in general. walking more instead of driving also helps. lots of small exercise can make a big difference.
if you're fat and need to lose weight, changing your diet can go a long ways. don't use diet pills and stupid stuff, just eat less candy and ice cream and stuff like that and eat healthy meals. it's easy when you have a mom or dad that cooks for you, but when you don't... i guess you could do it yourself or avoid eating too much or too unhealthy foods when you go out to eat.
if by fit, you mean having good endurance or being strong, then you need to either spend time running or lifting weights. running a mile or so a day is a good habit. if you can't run that far, start by running shorter distances, walking for a bit and running some more. it is best to set goals, like i'll run to the end of that block, to that light pole, so you know that you're making progress.