A note of caution, if you're playing in the Americas on an NTSC game, make sure you do Mission 1 first. Otherwise there's a known bug which can stop you reaching 100% complete.
After you've ensured you've done Mission 1, you can do Mission 4 repeatedly to generate some quick credits. It takes about 20s for each attempt, it's very easy to pass and you'll get 1,000Cr for each pass. Do this for 15 minutes, about 2 tries every minute, and you'll have 30,000Cr.
However, you will need to have passed A, B, and iB licence tests in order to enter the first 10 missions.
If you pass these licences, all Bronze, you'll win 3 cars anyway, a Volkswagen Lupo 1.4 '02, a Pontiac Sunfire GXP Concept '02 (both of which you can race) and finally a Nike One 2022 which looks cool, but you can't race in Simulation mode.
May I also suggest that you download the printable car list, prize list and race requirements list which is in this post?
Follow the instructions in the post to print it in the correct format for a compact all-you-need-to-know guide to what-to-find-where?
GT4_Rule
I sold my Lupo from B License test....
That shouldn't matter, you can purchase the VW Lupo 1.4 '02 new from the VW dealer in Germany for 15,490Cr. Just be careful not to sell any cars which have a value listed as ------Cr as these are usually only available once and cannot be purchased. Take time to read the side-scrolling text under the prizecar screen, which will usually tell you if a car is only available to be won once!