This guide will show you how to set a personal signature:
First, under My Settings in the Settings tab, select Signatures:
Once here, select the signatures tab indicated below:
Hitting the button labelled "Add" a popup will show that will give you some options for your signature:
Give your signature a name that can be distinguished from others you might create, then add your signature in the body section.
Once you're finished click save.
After doing this, your signature will show up when creating a new message:
This drop-down box will contain any signatures you create. This includes company wide signatures setup for use.