Well, first you have to have at least 50 posts (I think that's still the rule), then you go to your User CP and click 'Edit Avatar' (It will be on the left side of your screen, under Control Panel). From there, you can choose one of the premade avatars, or you can upload one from the internet, providing it is not larger than 125 X 125.
Try to spell better and use correct grammar in your posts, the forum rules do outline this and you should read them before posting. Grammar and Spelling are a hotspot at the moment. If you find it a problem copy and paste your post into MSWORD or a program with a spell/grammar check and then post it after.