I agree too with the Pilates and Fitball. If you can get out of the house, try to do a few intro sessions at a proper pilates studio. They will show you exactly how to recognise those core muscles we never knew we had and teach you how to work your pelvic floor and transverse adominal so you can do the techniques with proper form. Double leg lifts and table tops are intermediate/advanced pilates moves and I wouldn't recommend them if you are just starting out or haven't done pilates for a long time. A few one on one sessions on the pilates reformers are excellent!