Basically, go to "Settings" (found in the personal menu in the top right corner) and then click "Edit Signature" in the menu to the left. Make sure your signature follows the guidelines (500 px width times 100 px height by 50Kb) before using it.
What Tetris said applies. At 10 posts you can start to use Signatures. This is however no reason to spam just to increase your post count. I suggest (as a new user) that you read through the guidelines found here: http://www.mmo-champion.com/threads/...ore-posting%29
As this isn't related to Computers, I will go ahead and lock this. If you have further question related to this I suggest that you PM a Moderator or an Administrator.