Alright, first of all how do you choose your avatar? The avatar is the picture that will appear under your name in your posts and profile. You don't have to choose an avatar, but if you wish to do it, here is how:
AVATAR
Go to your Profile (it's on the bar menu under the banner)
You will see severl tabs listed on the top right corner of the profile window, click on Avatar
Now choose a picture from your computer (remember its width cannot exceed 150 pixels, the height cannot exceed 200 pixels, and the file size cannot be higher than 64 KB, but sometime it will resize it for you)
You can also put in the URL of a picture you already have somewhere online, OR you can click on Show gallery and choose one that is available
SIGNATURE
The tab before Avatar is Signature, click on it.
A window will appear, and you can enter text, links, videos, pictures and smilies.
You can also use HTML or create your signature elsewhere (there are many websites where you can create free signatures) and just copy past the code here
Click on Preview to see what you are doing. Save when done.
NOTE! Make sure you check the Attach signature box when writing a post or it will not show up.
Remember pictures and videos must be linked from outside, you cannot upload them.
The signature will automatically appear under all your posts.
Have fun!