After completing the mission(Forest Tower), this should appear to your screen. You are shown 3 clans (Uchiha, Uzumaki and Senju) and you can only choose one. The system will recommend one of the clans, if you join the recommended one, you will get a Gift Pack. (This is to encourage people to join the clan with the least members, so that it would make it more balanced.) You cannot change clan. All clans are visually the same and no special stats will be obtained from either.