Go to your UserCP
Choose Edit Avatar
Then either upload from your computer or reference a URL of the avatar you want.
PM me if you need more help.
I just checked your status and it says you joined the 10th of May, per the rules you have to had been here for 30 days and achieved 100 posts to get to community status. At that point you can change your avatar. If on June 11th you still can't change it let me know and I will fix your status.
Quote from Rules:
Achieved after 30 days AND 100 posts.