Need to gain muscle NOT fat!
You CANNOT gain muscle and not weight. Muscle weighs more than fat. Do you want to bulk or tone?
Gaining muscle has two big factors. Food and training. If you can give some insight on them both i and others can give pointers!
Get u a jumprope! Jumproping will help u get extremely tone and ripped. Do 12mins total 1 min fast 1 min slow. Moutain climbs really hit the lower abs hard. Also do some 40 yards spirts. But dont do cardio everyday best to do them at a beginning of your abs routine
There's a lot of advice flying around here (not all of it I agree with) & I'm reluctant to 'muddy the water' further, so I'll keep this simple. Calisthenics on the whole will tone and give you great definition, so will low-weight high-rep workouts. Avoid high-weight low-rep exercise as this will increase muscle bulk. A lot of guys giving you advice here are more the body-building type - which isn't what you want to do. :)