How much cardio should I be doing a day?

I really want to get fit I am 15 so and I do about 2 hours of hard cardio (the step machine) a day and it doesn't seem to be doing anything. I've been doing this for about 3 weeks

you might not see a loss in weight but go by how your clothes feel do they feel loser, larger. Another thing is that you might be building up muscle. you might not see results right away especially if you are unfit now. Give it a few more weeks ... Don't get discouraged its hard work to get in shape!

your body is most likely not eating the fat but instead the muscle you need to drink a lot of water and eat protein right after you work out so your body has something to break down and then it will breakdown your fat thus giving you endorphins because of the sudden release of sugar

You won't see results right away, try and give it some time (like a month or two). Or start doing some core exercises which will help for the fit, you're looking for. Also it may help you start seeing results faster...
Or on different days mix it up a bit, a huge part of physical fitness is keeping your body guessing so it doesn't get used to one certain type of exercise or you'll hit a plateau, and that is when your body is used to the same thing day in and day out and it's really not helping you... So try and mix things up with some pushups and other basic exercises.