Wow you really got some ridiculous answers here.
What you need to do is eat more and gain weight first. If you truly CANNOT get to a gym, find a good bodyweight routine. You can find crossfit routines or Death by Bodyweight (google it), which are great. Otherwise, a gym would be an awesome place to start. As for cardio, in your case, I would recommend more
HIIT than LISS or MISS because it
might help promote muscle gain and will certainly burn fat.
As for the diet, make sure you're eating clean. You obviously have fat covering your abs and eating crap isn't going to help that at all. Go to freedieting.com, find out your maintenance calories, and add some to gain weight. You can start with 300-400 and see the progress you make. If you don't gain, add more, if you gain to much, subtract, etc. If you're gaining much more than 1lb/week it's probably not muscle (though everyone varies).
Don't buy any stupid crap ab machines or ab workouts or diets, don't do insane amounts of cardio as that won't help you at all, and don't start doing thousands of crunches thinking that will improve your abs and burn your stomach fat. It doesn't work that way.