First, decide what you want the car to look like. 17's or 18's etc? How low do you want to go? What kind of wheel design will you look for, like 5-spoke or 6-spoke etc.?

If you're planning to change suspension and wheels, I'd advise on doing the suspension first because that will affect the parameters for perfect fitment.

Once you've lowered the car, use a calculator to find out what size will get you the fitment you want. An example would be something like this:

http://www.rimsntires.com/specs.jsp

Input your current setup on the left, then go and take some measurements for the kind of fitment you want. Play around with the settings in the calculator on the right to match those measurements, and you'll have an idea of what you'll need to look for.

Also check what others have done and ask them for their measurements (e.g., wheel threads or car meets).

It takes a bit of effort but the results are totally worth it if you can get it right.