How can i get a text signature on my phone?

I have the same phone, if you have Sprint the features should already be on the phone. Before you send a text, select the settings and there should be an option for signatures. If you have virgin mobile, the feature is not one the phone. You have to download a text messaging app. I downloaded GO SMS, I really like it. Once you've downloaded the app, you can go through the same steps to find the signature.