Sign in to https://rewards.forzamotorsport.net and claim your reward depending on your forza history. I was Tier 4, so I got 4 million credits. Then if you are looking for easy XP, just get a very fast car (like Lotus E21)... drive 20 laps on Indianapolis Oval. I think this gets you about 20-25,000xp per session.
Go to your messages on your game. You should see a message giving you money. Click "A" to collect. Same thing when you get cars for gifts.