Welcome to the forums :3 It's always nice to have new people around.
To edit your avatar (that's the one that actually shows up by posts, profile picture is just on your profile page):
1. Click settings on the top right of the website.
2. On the left is a series of options, including "Edit Avatar", click that.
3. Either upload an avatar or copy and paste in the link of another one. For example, I just copy and pasted this link: http://orig04.deviantart.net/9a29/f/...ta-d75ehpf.png
If you're just pasting a link, make sure to right click > view image (/the equivalent of that if you're on chrome or safari) and then copy that link and not the one to the page.
In those settings you can also edit your signature, but that's a bit more complicated, since you need to use the bbcode to add images.