hi, you need to buy a Bluetooth GPS receiver and some software e.g. TomTom. The software and reciever can be bought from Ebay for fairly cheap. (or software can be downloaded from various places - but this is illegal).
You install the software onto the phone then pair the phone with the reciever. You then have Sat Nav! To be honest, its a bit of a carry on. I have an N70 and the screen isnt the biggest. I'd just go buy a Sat Nav system.
Although if your wanting sat nav on your phone then hope this helps!