I want to start customizing my car where should I start? What else is a good idea to add without the possibility of harming the car? What size wheels will fit the car perfectly?

Try looking here: http://www.ft86club.com/forums/forumdisplay.php?f=23

Always start with what contacts the road first & that's tires. A great set of tires on these cars makes them amazing allowing you to be able to use all the available torque & corner like nothing else on the road. Once you are used to that avoid changing the suspension too much or you'll upset the factory balance that makes this car perfect & corner faster than all other high performance set ups. If you want more power stay reserved & improve efficiency with exhaust & intake systems for a more active soundtrack & a performance gain.