as long as it's easy and you can do it regularly - every day, if possible. you then slowly increase the amount you do over a period of weeks and months, adjusting it so you don't get more tired. it can be as little as walking from one room to another.
over time, you can increase the amount of time you exercise (or distance you walk), aiming to eventually do half an hour a day (you may need to divide this half hour into several short periods of time).
walking is the easiest exercise to try, but anything that you enjoy will do.
many people like to swim or cycle regularly. what you 