To add a signature to a post you must first create one via your User Control Panel. Once created, you can check the Attach a signature box on the posting form to add your signature. You can also add a signature by default to all your posts by checking the appropriate radio button in the User Control Panel. If you do so, you can still prevent a signature being added to individual posts by un-checking the add signature box within the posting form.
Note: you can also change this in your site profile as well.