Go to the track and start testing! It's the best way to figure out how it works and what serves your driving style... You can pick up some things around here, but all theoretical, just drive! Try the highest ride height front and rear and write down laptimes, then do the same with the lowest ride height... if the car feels bad, just change the ride height! Increase both spring rates 5.0 points and take the car on the track! Feels fastern and drivers better? Lower the spring rates again... Oversteer? Understeer? Toe angle try rear between -0.3 (oversteer) and +0.3 (stability/understeer)... front it's almost the same -0.3(better turn in) +0.3 (understeer on entry/oversteer on exit)...
If you're looking for tunes then you're on a good way, you're even in the tuning forum! GTPlanet is probably the biggest GT5 tune collection in the web!