To get an avatar, you go to 'My account' (which is next to your trade link to the top left of any CS page) then go to 'profile' (which is the third little tab at the top of the first 'my account' page). Then go to the bar on the left, and 'edit avatar' is the third and bottom bar. Click that.
Alternatively, you can just go
here directly.
You are then shown your current avatar, if you have one, and beneath that is a button which says 'Pick from our avatar gallery'. That will let you choose one of the premade avatars that CS has on offer, which is easy and there is a huge choice. c:
Also, you can upload a picture from your computer (remember, it has to be 100x100 or smaller), or you can use a picture from the Internet (though keep in mind most images are not for free use and belong to someone else!) by using the image URL of that picture. If you're not sure how to get an image's URL check
here for the browser you're using.