What do you adjust?

So had my second race as a premium member last night, just like the first i really enjoyed it but still raced really terrible lol.

I know i may be able to shave of a few extra seconds if i had a idea about car setups. Now i am going to look into it myself via YouTube etc because i would rather learn and understand what i am changing and how it effects my driving. I was just wondering to start me off what are the main settings on the setup screen to look into?

Cheers.