Drive around everywhere, every time you discover an Outpost, do all the PR challenges.
Just doing all the PR Stunts will drive up your "Popularity" level and all those challenges have big credit bonuses.
Also, work on the online achievements ... do a few Beginner races until you get past that level, then do the Playground list until you get the 5 of each game-type, and then just race until you get to 25.
Finally, get a buddy (or more), and jump into a Free Roam Co-Op game and do at least 10 of the Co-Op Challenges ...
You do all of the above ... and you'll have 3+ million credits and a bunch of extra cars easily before you even get past the Yellow races and if you keep your eyes peeled, you'll probably have a 50-85% discount on all parts and the achievement for driving on every road ...
Getting 1000/1000 in this game is so easy it's kind of hilarious. --- Matthew 5:5 - Blessed are the geeks, for they shall inherit Middlearth. You think you have lag? It took Jesus three days to respawn!
The final race, the one for the Horizon Winner, put all your assists off. I raced on hard difficulty, with a 120% money given, and the reward for that race is 5 million. Having assists of double that, I won 10,000,000cR from that race. You can also just grind street races, or like everybody is saying, go in Playground games.
GT: Ace 0f Clubz <-- 0f has a zero. 11 Unicorns. Mines R32 next.