So what you want to do is click on you username in the top right corner. Click on the drop down then do to My Profile. Then after that, click Edit my profile. Then on the left hand side there is a section called 'signature', it should be the fifth one down. Click on that and then you can change your signature.
If you want to use an image for a signature (like I've got.) then you'll need to use a site called Imgur, upload your chosen signature to there, then selected the link that's called the BBCode, and past that link in the signature box.
SpectraDust's post in Signature Location was marked as the answer