The most important thing you need to learn is that pedal next to the gas is called a break, use it.

Most people on these servers do not know about that pedal unfortunately.

They think the rear of your car is there to slow them down enough to make the corner. Always remember it is you and only you that can keep from hitting the car in front of you, and at no time should the front of your car make contact to the rear of the car in front of you.
I would run some races against a full field of AI. That should get you use to the "be prepared to be punted if you are a hair slower in a corner than the guy behind ya, for what ever reason". People say that the AI is bad in this game, well after the week end I had on the stcc servers "but this goes for any pick up servers" I would say they are dead on behavior wise to "human" mentality. But even with this mentality I would suggest you use the stcc to learn how people "race <-- I use this term lightly" here in pick up servers. Just remember "hot lapping" is not racing.