Originally Posted by jsconyers

This is a pretty frequently asked question. You would have found the answer quicker by searching.

How do I edit the "Sent from my BlackBerry®..." message? - BlackBerryFAQ

That will only work for email. For texts, you cannot put an automatic signature.
What you can do is create and autotext entry. Go into Options > AutoText, press the menu key, and select new.

Replace:
zx
With
(insert signature here)

Then at the end of every text you could manually type zx and it would replace it with your signature.