If you refer to an avatar, wherever you like; find a picture of someone you want representing your character and upload it somewhere. If you poke around in the Classifieds on the right menu you will also find people advertising their own images in the Artwork and Hosting section.
A sig? Same way. Both are added under Character -> Edit (character)'s Profile. Use HTML to add it; available tags can be seen to the right of the textbox when you reply to any post on these forums.
Use HTML, as I told you, and enter it into the textbox on your character profile page. I have already explained where you can find the HTML tags that you can use. If you are having trouble with this you will need to explain what problems you are having, or what it is you do not understand.
Use the codes displayed under the 'HTML ON' section to the right of the reply box when you post on these forums. The one next to the word "image" can be used in your signature field.