Updated 02/24/2004 5:38 PM
Gears - in general, just use the auto gears to adjust. Move the slider to the left and the car has more torque and accelration but slower top speed, move it to the right and the car has less torque & acceleration but higher top speed.
Take the car out on a track, when running along the fastest section of the track, usually the straight, make sure at the end of the straight, the 6th gear is only just at the beginning of the redline.
Suspension is a lot more complex to explain here. I'll have to point you to the tuning guide links on the right of this site. And, I have written various "tuning guides" in the comments of various setups.
Check out my Mitsubishi Lancer VII Rally prototype for a rally car setup.
In my setup for the Audi S4 and the GTR Vspec II R34 and the Mines GT-R - N1 V-spec(R34,J) for individual settings and it's affects. In ChevyCorvette's Opel Astra setup for Toe explanations. In DarkVenom666s Subaru Impreza Rally Car setup about camber explanations.
That should get you started.