You play as a she-cat born as Ashkit. You don't always end up as leader, you can simply be a warrior or medicine cat. The four clans are: Moonclan, Cloudclan, Sunclan and Lakeclan.
Also, just a sidenote. In part two, it'll take place in the Battle Arc, so if you'd like to know the full story, it'll say in the beginning of Part two.