How often and what you feed your fish depends on what type of fish and the size of the fish that you have. It may take some experimenting until you find the appropriate amount of food for your fish, as you do not want to overfeed your fish.
You can test out meal sizes by sprinkling a small portion of food into your fish’s bowl or tank and observing. Your fish should eat the food (or as much of the food as it can) within a few minutes. Most fish only need to be fed once a day, but you might prefer to feed your fish two small meals per day instead.