How to get a slim tummy?!

I'm a 14 year old girl. My whole family has a metabolism problem, so we have to be really careful what we eat and how much or we gain weight. I'm not fat, I'm average. But in the past few years I started going to an online school ( I work on a laptop at home ) and I'm sitting a lot. I'm not really physically active. I live in Oregon so it also rains ALL THE TIME! and I'm unable to go outside. I have five other siblings so moneys really tight. (Jsyk) Anyway, I've had a bit of a tummy problem the past few years, ( probably also because of puberty ) And can't seem to get rid of it. It isn't a huge deal but it does prevent me from wearing a lot of clothes and swim suits. I also am getting big thighs with lots of stretch marks that won't go away. I do crunches and try to eat healthy.. but its not really working out. I'm tired of being embarrassed and sucking in my tummy. My mom and dad like me to be really modest ( so do I) but its getting to be a real hassle to go shopping anymore because I find so many shirts that would fit well if I didn't have such a big tummy. All the fat is mainly at the bottom by my belly button ( which I might add I can't suck in very well ) I'm not really sure what to do. I'm definitely not going to starve myself or make me throw up ( couldn't do either one even if I wanted to!) Because that just incredibly stupid.
I just don't know what to do anymore.