Find a sport that you enjoy. It seems so obvious, but I do believe it is important that you like what you are doing. I dislike running with the same degree of intensity that I despise certain vegetables, so I don’t run. I do, however, love to play squash and I could play all day if I had the time.
Being part of a group can drive your motivation. As a member of a group you are often more secure, and they can share in your goals and care about your progress.