You can change the signature that appears below your topic posts, add an image to use as your avatar, and more by editing your Profile.
Start by logging in to both the ABC web site and also the Forum.
Then click the Profile tab near the top of the page.
Once there, click the small Edit icon in the blue bar, all the way to the right near the top of the page.
You'll find several tabs that will give you access to your account settings and much more. It's all pretty self explanatory, so just explore and adjust the settings to your liking. You can always come back and reset them if you don't like what you've done.