Here's all the rules
Honestly, don't do much your first year, since it really won't matter anyway. Find out precisely how to handle your car in stock form, then you'll be able to figure out what aspects of the car you would like to improve through modification. The car will remain much faster than you for quite a while. Try to go to an autoX school, like Evolution, at the end of the year. You'll be 100% shocked at how much faster it'll make you. Also, read the sticky in this forum on AutoX.
Good tires are the best thing to start with, though. Get some summer-only max performance tires like Kalken Azenis. Best performance upgrade you'll do short of a full turbo kit.