Take care. The best way to avoid obesity is to take care of yourself. There are certain factors that are difficult to control, such as poverty, ethnicity, and your level of education, but certainly using more calories than you take in is the simplest formula. That is, be physically active and make sure to eat moderate portions of healthy foods. For more tips, see the cdc website and your provider.
Eat less, & exercise. Obesity can be prevented or overcome by a permanent change in lifestyle and in personal outlook. If a person eats fewer calories than he needs to live on each day, his body will use the stored fuel (fat). If a person exercises more than he did before, but does not start eating more calories, then his body will use up the fat (stored fuel). Easier said than done, but definitely possible to do!