Do a lot of aerobic exercises such as running, jogging, swimming, biking, etc. If where you live doesn't have such space, try jumping jacks, squats, and "prison workouts" (stand up straight, get down on your palms, push your legs back, pull them back in, stand straight again. Not a good explanation but it gives you an idea). Whatever you enjoy doing. Do it for a long time, and continue to do more after each work out (like say one day you run a mile, the next day try to go for a mile and 1/4, then a mile and 1/2, etc).
Keep in mind that if you decide to take up on running or jogging, I highly reccomend doing it on a track field or something. Your feet pounding against cement pavement for a prolonged period isn't healthy for you.
You Might also want to do several sit ups. I would stay off pushups and pull ups for now because you're trying to get rid of that "beer belly", but if you're trying to build your arms and chest, go for it. Don't work out with weights either. Only work out with weights if you plan on building mass.
And of course, eat right. Put away all the junk food. Don't even drink juice. Stick with normal water frm the tap or bottled water. Drink 8 cups daily. Try to incoperate a lot of protein into your diet, and be sure to eat food and not starve yourself by eating nothing but lettuce and salds (they lack proper vitamins and minerals, lettuce especially which is nothing more than fiber + water).