Kvdb:
if i put the device in a car, i'd like to know where that car is on any moment of the day.I'd like to send the gpsdata via wifi, i suppose i will need a WifiShield for that.
So your car will never leave the range of your WiFi network? If that's the case you could just look out the window to see where the car is.
Most trackers use the cellular telephone system for communications. You can eliminate the Arduino by using a cheap Pay As You Go cellphone and install tracking software.