You don’t grow abs.
Well, actually, you do... but I’m pretty sure mean sculpting them... as you should really already have your abdominal muscles inside of you...
You eat well.
You sleep well.
You work out.
You mix cardio and strength training.
You do crunches – they’re more effective than sit ups, and less strenous on your neck and back.
You stretch.
You should also probably have a professional help you out with it all, because you don’t want to hurt yourself. I don’t mean a PT (though it wouldn’t go amiss), but just a fitness test with either your doctor or a trainer at your local gym, though, the latter might require you to actually join said gym...
Everyone’s different, but being healthy overall should kick start you.
A lot of other stuff comes before abs.