If you are a gmail user, just click on the gearwheel icon (like the one above) toward the top right and then click on Settings. If you scroll down you will find something called Signature. Then click the button below No Signature and put the signature you want in the box using the tools you'll find there.
Click save changes at the bottom and you're good to go!
Now I just need to figure out how to do something similar in comments on blogs... any ideas?