Planks. Try planks. You hold yourself as if you were going to do push ups but don't go down. Hold as long as you can, then you can work on side planks to define the obliques.
Wt loss, exercise. Get to ideal body weight which is 110 lbs for first 5 feet tall and then allow 6 more lbs for each inch taller than that for males. Daily routine is: lie on carpet or pad, place a 3 inch padded tube under your lower back, keep feet raised off of floor, then do 50 sit ups (meaning touch shoulder blades on floor and then flex spine as much as possible, then repeat. That is, move spine, not hips).