how can i become healthier and tone up my stomach

Im 22 and as far as i can remember i have always been underweight and still am.  I use to have a flat stomach but over the last couple of years i have started to put abit of a belly on and people are alsways askin me when i am due. I can be quite active and i run a mile and a half twice a week and also everymorning do 100 sit ups and 20 press ups and also do it at night ive been doing this for about six months and there has been no difference.  I am quite a fussy eater and my down full is snacking alot i always seem hungry i dnt put the weight on but it shows around my stomach.  i am trying to stop smoking i have the odd one here and there and have cut out fizzy drinks and also reduced my alchol intake.