You can create a character by going to the 'Biography' board and post there with your character information, but that is optional. To roleplay in a clan, there's no need to speak to the leader if you're character was born in that clan. However, there have been character bans in some of the clans that it is required to speak with leader in order to join the clan if your character was originally from another faction. Take a look at the news here:
http://warriorcatsonline.com/forums/...ead.php?t=1091
To post, there is no need to start a new thread. Post in the threads that staff have created for you. At the bottom of the list of posts is a blue button saying 'reply' and type out your character's actions. To reply to specific post by a writer/poster, press 'quote' to catch the poster's attention that you have replied to their post. Make sure to read the rules, guidelines, and character bans thoroughly. Hope that was helpful!
Also just a note, try and spell out your words and use a little grammar. Also read the 'site assistance guide' if you can...