I see that you are under 18. Us under 18- ers worry about this stuff all the time but here is what you need to know: If that little belly fat of yours is a deal breaker for her... she is probably not worth it... but I just have this feeling (even though i don't know anything about you two) that she probably won't mind as much as you seem to.
I wouldn't mind a guy with a little belly fat at all. That's not something that would be important to me.
No one is ever perfect, no matter how much excercise they do. But on a side note, if you still want to make an improvement just choose two exercises geared towards your abs and work on them for about 15 minutes each day alternating the two exercises. It'll make you feel better about yourself if you physically put in a little bit of effort each day :)
If she likes you, it shouldn't matter if you have some "chub". Personally I don't mind what a guy looks like, it's the personality that attracts me to a person. Looks are just a bonus. My boyfriend has a little chub on him and to me it just makes him more comfy to lay on.
If someone doesn't want you because you are not perfect, they are shallow assholes and you should get rid of them. Nobody is perfect and that is not why you stay with someone let alone fall in love with them. I am 38 and have had a baby - I am in good shape but am not perfect. Any man that doesn't like my imperfections can take a flying leap. I am sure this girl that likes you isn't perfect herself and she is probably worrying about what you will think about her!