Winter tire size?
 

What size winter tires should i get for stock wheels, and when should i buy em?

i guess F-> 225/50/ and R-> 225/55 if you have stock 17" wheels. you want the tires to be as narrow as possible.

What do those numbers mean anyway? Where can i learn about that? Also, i've got the stock 18's.

they represent the tire size. If you want to know what tire size to get for winter, well, you are restricted by your wheel size unless you are getting new wheels.

Go to tirerack.com all sorts of good info there

Alright, so which exact size do i need for the stock 18's? Same for front and rear?

Check out tirerack.com. Just punch in your vehicle's info and what you're looking to get, in this case Winter Tires. They also have packaged deals where you can get both tires and wheels at a fairly reasonable price.
I just bought some 17" Dunlop M3 along w/ some cheapo wheels even though my stock wheels and tires are 18". Don't want anything too nice that'll get eaten up by the winter road salt.
Good luck!