Keep the settings at default, you can always change it later. Make sure power and USB are connected properly. You can either connect USB whenever you're PS3 is on or off, it does not matter.
After you plug in, turn on the PS3 or if its already on, the wheel should AUTO calibrate, make sure you do not touch the wheel.
Then just turn on GT5P, and enjoy. To turn on CLUTCH, remeber only cars that have clutch will work, press the TRIANGLE button on G25 *very top black button* once you start to race and take control of your car. To deactivate clutch you need to press it again, and you need to press it every single race to turn Clutch on, its annoying at first but you will get use to it.
Oh and remeber, you can only use clutch when you have your G25 set to H gear rather then sequential. To switch from sequential to H shifter, press down on stick shift and turn knob.
Enjoy!
If the steering still does not work... make sure you properly got all the cables hooked on. And I don't know how many controllers you can have turned on, but dont hvae more then 1 on.