If you work out aerobically, like taking a brisk walk for one hour each day, you will lose weight all over your body. This is actually more effective than trying to focus your workout on one isolated "problem" area, like the tire around your middle or your outer thighs. With that said, doing sit-ups and leg lifts in addition to aerobic exercise will help build muscle in those particular areas. Be sure to stretch properly and work the muscles opposite those areas (namely, inner thighs and lower back) to help avoid injury.