count your calories, and know your body because your body burns a certain amount of calories a day so if you eat over that and don't do anything extra to burn that extra off you will gain for that day. So exercise and watching what you eat is important, but like I said you have to do the research because you can run a mile or walk 2 miles and burn maybe 120 calories but it a candy bar and pick up 300 to 400 calories or more depending on the size so you have to work do your research and you will be fine.