Look For Credits
Levels are based entirely on the credits you have won while racing. Races are also restricted based on your level, so you won't have access to everything when you first start. Begin by selecting the races that will earn you the most credits if you win and do those first. This will give you the most credits for the least racing, although you may end up racing against strong opponents in a weaker car. Make sure you have the skill to keep up!
Stay Away From The Garage
While spending credits does not hurt your ability to level up, it will reduce the power you have to buy cars and parts when you need them. As you progress through the game you will have to race manufacturer- and model-specific races. Keep your credits in reserve so you have them ready when you need to buy a car to earn the big credits.
Look At The Cars You'll Win
Look at the cars you'll win for each event. Many of the events will reward you with a car for winning the event. Look at this car in advance and pick the series that will give you a car to race the next series. This will save credits which you can then spend on upgrades to make the car go faster and therefore more likely to win.
Cheat
There is a cheat for extra credits in "Forza." While this won't push you up the levels it does give you enough money to buy whichever car you need, along with all the upgrades you need to make it faster than the opposition. To get the maximum number of credits select "new profile" and type your name in as "tEAm4za." This will give you the maximum number of credits but you'll have to start working your way up from Level 1 again.