Welcome to iRacing.
You're embarking on something that closely resembles a real racing career by participating in the iRacing community.
Although it might not the first thing you wish to do, take the time to read through the sporting code which will give you an idea about what to expect in terms of how to be a competetive and clean driver, as well as containing crucial information about what it takes to earn higher licenses.
You'll be starting out in the rookie Solstice on tracks such as Lime Rock Park, Summit Point, Laguna Seca etc. and I'd advice you to learn the car and the tracks as well as you possibly can, where your braking zones are, what gears to use, how to make sure your entry speed into corners will assure that you will be able to make the corners and not go wide, what works and what doesn't.
I've got a feature article coming up some time "soon" I hope about driving the Solstice around LRP. From that article you will see that the subject matter is so comprehensive that it is impossible to give general guidelines and you will probably have to spend some time honing your driving skills before being able to actually feel what the car is telling you on track, and before you'll get down to really fast lap times.
Look through the Fast lap series that Chunx and I did for SimHQ. Although the sims used in those articles are rFactor, GTR2 etc. you'll be able to learn about lines, traction circles and other fascinating stuff!
http://www.simhq.com/_motorsports/motorsports.html#featuresLook for videos and track guides on forums and study the drivers that are quicker than you and try to understand where and why they are quicker. Try to learn which lines work best and be ready to experiment in a practice session where you can try out your ideas when you are alone on the track, not endangering your progression by picking up incident points. Also find out how you can drive off the line in case you have to pass someone or be passed.
Read some books about how to drive fast, e.g. Ross Bentley's excellent "Speed Secrets".
Drive within you current skill levels in races and if someone is clearly faster than you, let them by. Do not block, don't weave around but hold your line. If you are found blocking or weaving, this might lead to someone rightfully filing a protest against your driving.
Be prepared for the unexpected. Behind a blind curve there may well be a rookie racer who's spun out and is now located right in the middle of where you planned to be going. On ovals the curves might not be blind, but things can go pear-shaped very fast indeed and taking a quick and hopefully correct decision regarding avoiding action may well be the difference between cooming through unscathed or taking a hit.
"If in doubt, stay out", so only intiate passing attempts that you are absolutely confindent about being able to pull off. Give the other guys and gals room to race you as well. If you put yourself and the other racer in a situation where the other driver is forced off line in order to avoid hitting you, it is a bad situation.
Don't copy real world driver tactics as some of them do have attitude problems, and after all this is not racing for the world championship but intended to provide a good and enjoyable experience for all participants.
Enjoy the racing, be a clean racer, finishing races is far more important than crashing out while trying to make up for lost time by going like a rocket-ship.