Buy a Toyota Integra Type-R and upgrade it to the maximum. Now go to the Type-R meeting in the begginer leauge. Do the series race (5 races) and you get 5,000 for every race and 10,000 at the end. 35,000 already! Now you get an Acura worth 75,000. Sell this and you have gained 110,000 credits in 30 minutes. Easy money!