don't do sit-ups...do crunches. and make sure you use slow, proper form with those crunches.
if you're using proper form and contracting tight enough, you'll be barely be able to do 25-30 (a newbie probably won't be able to do that even)...
but THAT IS FINE...you don't need to do anymore than, say, 25...as long as you're using proper form. by 20, your abs will be burning.
a trick that i learned and has worked really well for me is to do abs BEFORE the rest of your workout....
most people do abs afterwards and by then they're so tired they cheat and use bad form and don't work them efficiently.
try working abs FIRST and then work out with weights/cardio...you'll notice a HUGE difference in less than a month!
but more importantly than working abs, is proper diet and cardio. cut out carbs late in the day (and alcohol, of course) and do cardio at least 3-4 times per week or else you can do all the ab work in the world and you won't see results.