That depends if you are going to airbrush or hand brush. If it's airbrush then go
HERE and scroll down to MusicCity's excellent articles on airbrushing.
If it's hand brushing, then I would suggest starting out with NO thinner at all. Be forewarned that hand brushing Tamiya paints is often very frustrating for people. It dries super fast (alcohol based so it evaporates quickly!) so the trick is to use a very good brush (red sable is the best, but not absolutely required, just don't cheap out on brushes!), and to paint small areas. Tamiya self levels nicely so don't "overwork" the area you're painting. If you do experience problems (seemingly dry paint can quickly become reactivated with fresh paint and "roll" up) start adding thinner to the paint. Start with maybe 5 or 10%. Tamiya thinner is basically Isopropyl Alcohol with acrylic retarder to slow down the drying a bit.
The good thing about acrylic paint is if it does goes badly, immerse the piece in Windex (the blue kind with ammonia) and the paint comes right off!
Good luck and let us know how it goes!
Bill