After remembering about the search button, read the FAQs... no, I'm not kidding nor teasing you, GT3 was fairly obvious but GT4 is a nightmare for a beginner to figure out what to do.
Some basics:
- Run every race in both A-spec and B-spec. What are they? How do you win at them? RTFM, of course.
- Go to every car vendor, and make yourself a list of every race there that you can do with whatever license you have. This will simplify life.
- If you have no car, buy a used Miata, or just use the cars you win doing licenses.
- When you have enough money (after 25% completion), but a Chrysler Crossfire, win the Crossfire race and get a Viper. Upgrade it fully. Race El Capitan endurance and win a Toyota "Minolta". Rerun as needed (maybe at night) selling the "extra" Minoltas. Cash problem eliminated permanently.
- Before the 25% completion, do the same thing to race cash, by running the Costa D'Amalfi rally and selling the prize car... less cash, but plenty anyway, and quick.
That should get you started.