GT1 can be a bit hard for new players. So here's a walkthrough of some sort to get new players through the first parts of the game without much trouble:
1: First of all, before you do anything, go to the Toyota used car lot, and buy the Mk. III Supra for 7,250 credits (It will be the first car on the list, you can't miss it).
2: Now, go to the License Center and obtain the B and A licenses.
3: Now go back to Toyota and under "TRD" buy a sports muffler for your car.
4: Enter the Sunday Cup (under "GT League") with your car and win scoring the pole position on at least 2 races.
5: Go to "My Home" then "My Garage" and get in the prize car.
6: Go to the Mazda used car lot but instead of selecting "Buy", select "Sell".
7: Sell the prize car.
8: Get in the Mk. III Supra, and sell it at Toyota.
9: If you did everything right, you should have 44,300 credits now. Go to the Subaru new car lot and buy the Impreza Type R.
10: Under "STi". buy a racing muffler, stage 1 turbo, and racing intercooler.
11: Repeat steps 4-7 twice, but this time, score the pole on every race.
12: Sell the Impreza at Subaru.
13: Now you should have just under 100,000 credits. Go to the Dodge new car lot and buy the Viper GTS.
14: Enter the Normal Car Cup with it and win. You get 50,000 credits per race you win and 10,000 per pole. This will get you up to 300,000 credits, not counting the championship prize. The Viper should be enough to beat the Clubman Cup and GT Cup as well.
There you go. A nice head start. Just make sure you spend your credits wisely and don't buy a special model the first chance you get.