what kind of offset should i look for when getting wheels for my tC

that depends on the width of the wheel you getting

http://toy4two.home.mindspring.com/offset.html

i don't know what width i would get.

aight i'll give you an example then. i got some 18x8's with a +36mm offset. the factory wheels are 7" wide and +45mm offset. so with an 8" wide wheel and +45mm offset the rim would be split evenly 1/2" extra on either side. with the +36mm i got it shifts the wheel 9mm plus that 1/2" to the outside and 1/2" minus the 9mm to the strut side. its kinda confusing...thats what that calculator i posted is for. but there is alot of room to push the rim toward the outside on these cars so i would prolly recommend around a +35mm for 7-8" width wheels.