Low-level cardio is good for burning fat. Walking, biking, rowing, something that will increase your heartrate no more than 65% than your resting rate (typically 120-125 for 20-30 yr olds).
Unfortunately, the abs are the last area we lose fat from. The changes start at the extremities and work towards the core. However, do plently of upper/lower abs, obliques, and lower back exercises to focus and start toning that area.
Do regular crunches, then alternate side crunches (lie on back, knees bent, rest right calf on left thigh and bring left elbow to right knee, do the same for the other side)
For the lower abs, lie on back, hands to sides, lift both legs slightly off the ground and hold for 2-5 sec. OR lie on back, lift both legs slightly off ground and open them up to shoulder width and bring back together. repeat as above.
For lower back, lie on stomach, legs stretched out, put hands behind head and using abs, lift your shoulders off ground and do "backwards crunches".
For sides, stand with feet shoulder width apart, arms down to sides,holding weight (5-10 lbs) in one hand, bend to the side from the waist lowering weight towards your knee, use other hand to reach over head as you bend.
Some other great exercises are lunges, push-ups.
Challenging one for legs, thighs and butt: lie on floor on side one leg onto of another. Use the top leg to "spell" out the alphabet or numbers 1-25. The bigger the letters the more you will feel it.
(I've tried it and I can only get to letter K before the muscles burn)
Work at gym and workout regularly with personal trainers