Just make sure you watch what you eat, making sure you eat mostly vegetables, fruits, and lean protein, along with the more healthful fats, such as avocado, olive and coconut oil, etc if you're going to eat fat.
If you don't have time to join a gym, just take a few minutes several times/day and go for short, brisk walk. Even 10 minutes at a time is good. Take stairs instead of elevators, walk instead of driving or riding whenever possible, etc.
Anything that keeps you moving is helpful.