Can anyone tell me if i have to make any changes to my car if i want to upgrade from 15" to 17" or 18" rims on my RSX.

no, you don't have to make any changes, I also wasn't aware that RSX's came with rims smaller than 16's

dont need to go bigger than 16s as long as you lower the car man

Physical changes? Nope!
You only need to change something on your car if the tire rubs. This usually happens on lowered cars or wrong tire sizes. If that happens people usually just roll their fenders for more clearance under the fender.