Forza 3 startoff hints(Asia start):
If you play online(gold account)
To start off you can choose any of the starter cars(I chose the Fit just 'cause' I'm a Honda owner), 1st play a few races to gain approximately 6000cr(this can be done over and over on your favorite track in -> Go Race/Event List).
For this, I chose Asian Manufacturer Open/IIC Mini
Circuit Reverse(1st circuit). If you come in 1st, it'll
probably be 2-3 races for the money.
Then go to the auction house and purchase a Honda NSX-R
GT(you can usually find one cheap), between 1000-6000cr.
Next race in -> Honda Acura Owner's Meeting/TR Motegi West Circuit. Once you earn 10,000cr or so buy a 02 Nissan Mine's R34 Skyline GT-R
(with both the NSX-R GT & GT-R) you should be able to clear quite a few of the races for more money.
PS. From there I'd recommend the 08 Nissan #23 Xanavi Nismo GT-R as your workhorse for more credits. That one helped me get into the 100,000cr range for more car fun!
PPS. Before you upgrade your car, remember that the higher your car's level the cheaper the upgrade cost for parts are.
Spoiler: ALSO when buying from Auction House be sure to know what the Manufacturer's price is for the same car. Many people get suckered into overpaying for a 'used' car when they can just buy it cheaper 'new' lol