Welcome to Android Central! Are you talking about SMS text messages? Most texting apps don't have a signature option. If your stock messaging app doesn't have that, then you could look for a 3rd party texting app that has it. But I'm not sure if a signature would be appreciated by your text recipients -- it could be very distracting and repetitive to see the same signature on every single text message from you. Signatures make more sense in an email.