Here's a tutorial on how to display a signature and avatar on the forums.
Step 1: Go to "Profile" on the top.
Step 2: On the top, you will see a tab for "Signature" and "Avatar". Click on each depending on what you want to add/edit.
Step 3: Go to the "Preferences" tab after you have edited your signature. There, find "Always attach my signature". Make sure you have selected "YES".
And you're set!