there can be several reasons of obesity, but what I think are the most important causal actors of obesity, are (i) physical inactivity, and (ii) poor diet. It's important to replace your unhealthy diet with healthy one, such as limit the use of rice, bread, or sources of carbohydrates, and excessive use of oil.
For physical activity, in my opinion, walking or running is good, if you cannot go to the gym daily.