Welcome to GTP! The only dumb questions are ones you don't ask... There are a lot of helpful people here.
To tune your car, get in the car and start any race, or go to the Testing area. At the opening screen, where you have the option to 'Start', 'Qualify', etc., one of the options is 'Settings'. Click that button.
You'll see a number of selections, one being 'Settings' and one being 'Change Parts'. 'Settings' is where you adjust whatever is adjustable. On stock road cars, that won't be much. However, once you start buying upgrade parts and tires (at the Tune Shop in the simulation screen), there will be more things you can adjust.
'Change parts' is where you change tires (assuming you already bought different tires at the Tune Shop) or add/remove parts in order to adjust your car for a particular race.
Your settings changes are automatically saved to the car. The 'Save Settings' option is just if you want to save a settings file for a particular car for a particular track, and reload it later.
Hope this helps!