Just some questions that lots of people ask.
How do I get started?
Easiest way to get started is to start with the local track days and do some schools to get used to it. Autocross is popular for the small time racing for the weekends, or you could do some full track racing if you would rather do a full track rather than cones.
What are good Books for Racing advice?
Going Faster by Carl Lopez is one that Febi suggested. Personally I haven't been able to find any at the bookstore worth buying, so I'm still looking and I'll let you know when I find some.
Is the 318is a good track car?
Yes it is, with the weight ratio it allows it to come into corners much faster than other cars, even the 325i and such. With the gearing it allows it to keep up with in the bottom end cars with more horsepower, but it does lack some up top because of the low gears.
Where can I race my car?
Many people do autocross because it is just in parking lots and a weekend sport with classes for all levels of cars and mods put on them.
What class does my 318is belong in?
For autocross either the stock and street touring are the most common. These can be raced with a stock 318is and still allow it to be competitive.
I have a ton of mods... can I keep them?
Yes, you can race in some of the more prepared racing classes which can be both good and bad. You get to keep it modified, but your also racing against more prepared cars.
How much does it cost?
Autocross can cost not very much. To race at one scca event is usually 35 dollars. The more modified your car is, the more maintenance you may have to pay for, but there are classes for even stock cars which can be 35 dollars for the entry fee. Either way, if you modifiy or leave it stock, you will have to pay for maintenance which is normal stuff, so just depends upon what you do with your car.
What should I do to Prep for racing?
Change the oil, check all the fluids and take all the crap you got laying around in the car. Definately add a 1/2 quart over what the regular recommended amount is because of the sloshing around. If you add too much though it will burn oil but add a bit so that it can slosh around and still be sucking up an adequate amount of oil from the pan. Check tire pressure and also brake pads because it sucks having the brake pads be bare by the end of the day. Make sure your brake fluid is good and preferrably flushed very recently before the race so that it doesn't have moisture inside the lines.
Let me know any questions and I'll try my best to answer, or if you know of any other info, just post it up and I'll add it to the answers.
Brendan