hello, welcome to AFM!
ok, someone gave you horse-sh** for information.
firstly, you cannot put an 18-inch tire on a 17 inch rim.
secondly, your rims are 15x7 and will not fit a 17 or 18 inch tire.
3rdly, 17x9 & 18x10.5 are the size of the rim, not the tire.
and lastly, your tire size is 205/65/15
if your stock or close to stock, and looking for more traction, i recommend keeping your stock wheels/tires for now and installing a t-lock.
if you really want some wider tires on your stock 15" wheels, you can put 225/60/15 on the rear, theyre a little bit wider, i did that for a little while and they looked ok.
i am assuming your avatar picture is accurate; if so, good looking stang:bigthumbsup