If you don't know anything about tires, here is a quick explanation of at least what the numbers mean.
for the example: 225/40/R18
225 - Width in mm
40 - Sidewall height represented as percentage of width (40% on a 225 =90mm tall)
R - Radial
18 - Wheel diameter.
You do not want to put anything larger than a 225 width tire on the stock rims. And I would highly recommend not getting anything taller than a 40 sidewall on any tires larger than 225 if you want the thing to handle at all. To keep your overall diameter as close to stock while increasing the contact patch (keeping your speedometer correct) stick with a 225/40/r18.
I recently put zr speed rated 225/40 Nitto NT555 tires on my car. I bought them from tire racks e-bay store for $111 a piece. They are very nice tires for the price. While the straight line traction was very noticeably increased, you can feel a slight difference in flex around the corners. If I had it to do again, I would but r-compound 215/45 tires instead. But, that said, I am pretty happy with the results.