I’ve been playing Forza since Forza 3 and I’m still a newbie 
The game is very deep and it is very rewarding if you have the patience to learn it. Therein lies most of the problem, people don’t want to spend years learning to play a game.
The beauty of Forza is that it does attempt to simulate real driving, so anything you can find about real race car driving can apply to the game (at least on a fundamental level). Combine that with the level of dedication and expertise available on these boards and you can start tearing up the roads in no time.
Slower is Faster
The golden rule of racing. Racing is not about speed, racing is about control. If you cannot control your vehicle, then speed does not matter. If you feel you are losing control, gently let off the throttle and maybe apply the brakes (gently). If you think you’re coming into a turn too hot, you probably are. Slow down. Speed is the rabid dog on a leash. If your break the leash, you no longer have the dog.
Hold Your Line
Once you’ve picked a line, stay on it. This goes back to the fact that racing is control. Do not let speed change your line. Slow down, if you must, to hold the line. You are not the only one on the track, and it is a big no-no to hit another vehicle or go off the track.
Hit the Apex
The shortest distance between two points is a straight line. Therefore, the quickest path through a turn is through the apex. Whenever it is safe to do so, hit the apex. This is usually the shortest distance you can travel and the straightest as well. This allows you to maintain the highest speed and provides the fastest exit.
Don’t Be Greedy
Racing is a competition, and as such, has a code of sportsmanship. To avoid collision, the passer must always yield to the passee’s line. If you sneak into an apex just before the car ahead of you does, that is dangerous and greedy. Racing is not about stealing opportunities and cutthroat maneuvers. Racing is about finding the fastest and safest path. Never endanger another racer, and don’t take lines that you know could cause a collision.
“No race has ever been won in the first corner, but many have been lost there.”
Basically, be patient. Especially at the beginning of a race, the cars are all very close together and collisions are extremely likely. Take your time and wait for the crowd to disperse before trying any aggressive maneuvers.