There are probably other wheel size calculators out there, but this is the one I use:
Tire Size Calculator

Find out what your stock wheel size is and then chose the settings for the size you're thinking of. Just make sure your new wheel size is within about 1% of the stock size. Of course you have other things to take into account like offset and tire width.

This means you need to do a search to find what your stock offset is and what offsets are acceptable for your car.

And I don't recommend 18" wheels. I personally don't think it will look good, not to mention the added weight.