how can I download my text messages to my computer from my iPhone

Take a screenshot and send it via e-mail, maybe?

http://manuals.info.apple.com/en_US/iPhone_User_Guide.pdf

No need for screen shots. Enter the text message thread, tap "Edit" and then touch each entry that you want to receive. Select forward and enter your email address (instead of a phone number to text to). You will receive the discussion thread via Email. The drawback is there aren't any date/time stamps included with the emailed discussion thread.

could you please explain this a little better thanks i cant figure it out

Please help, it does not owrk on me, as the forwarding to my email address transforms the text message into an MMS that costs me a lot of money.

An easy but tedious way to download text messages from iPhone is copy the messages and paste on email, then send the email to yourself. Next you can see those messages on computer.