Hi CJ and welcome to MobileRead. Good to see you here.
To be able to set up a signature, there is a prerequisite of ten posts and one weeks membership. Once you have these then go to your user CP (there is a link to that in the blue bar at the top of this webpage). In the left column there is "Edit Signature" under the "Settings and Options" heading. Enter what you want in your signature and voila! Once you create a signature, every post from then on will have it in it. Previous posts will not have the signature added. Also, when or if you change your signature, a similar process....new posts will get the new signature, others stay as is.