Sprint Navigation (i.e., GPS Navigation by TeleNav) isn't available for the iPhone. But, you can use the TeleNav free version or pay version (more likely) which are available in the App Store.
Your phone comes with Maps (by Apple), which although in some areas it has some mapping and location bugs, still is a quite good program for turn by turn (voice guided) navigation. Although I live in a big city, I find it quite accurate and the interface convenient. Try it out. It's simple to use and understand, and works well (in my experience).
You only get the voice part of the navigation from tele nav if you pay for it, the free version does not come with voice . ,I would like to why I am paying for GPS thru sprint which is tele nav ,but have to pay for features that are free on other phones ,but because I have an iPhone. Special since they sale iPhones so tele nav with voice should automatically download ,when you set up your phone with all the features ,special if your asked to enter your pin