Welcome!
Start by reading the FAQ on both wheels and tires. Although a bit out of date it will answer many of your questions. Then consider not doing 18s all around unless you are more interested in looks than performance. At the moment, I consider the 2002 OEM sizes to be a very nice compromise between looks, performance and wheel/ tire selection.
Keep in mind that most wheels, even in the original sizes, are much heavier than the stock ones. By the time you move up to 18s it is very difficult not to end up with anchors at all corners. You may also encounter clearance problems on the front.