You may choose the clan you wish to be in. Any clan is free 24/7. Just write down what clan you want to be in and your in it. I must approve first though. If you can't decide your clan then go to this site to decide. http://www.warriorcats.com/warriorshell.html
If you got Windclan then you are in Skyclan.
If you got Riverclan then you are in Moonclan
If you got Thunderclan then you are in Forestclan.
Least, but not least if you got Shadowclan then you got Darkclan.
IF you don't wish to go on the site just pick which clan you want to join.