As for not gaining fat and keeping your abs, most/all types of cardio will cause muscle loss and hinder your growth. From what i know the least damaging cardio that will still burn your blubber away is HIIT(High Intesity Interval Training). Basically your sprinting almost full out for 1-2min, then jog 30secs and repeat over and over.
Thats what ive been told and heard works. Hope i could help, good luck.