Try to understand a manual transmission as early aged as possible. Like sombody said, the sooner, the better. You have more control over your car, as to when to shift, and when not to. You can also take advantage of the clutch feature (unelss thats in auto? ive never tryed it).
Heres one thing to know, you dont ALWAYS get the best acceleration when you shift at redline, or as high rpm as possible.
You want to shift, right after the powerband dies out (usualy before redlline), so that when you get into next gear, you can still land in the powerband. On some cars, you dont notice the difference, but with the XRG (the non turbo rear wheeld rive car in Demo), you notice it at the backstraight on the Backwood track.
Good luck. Ive never liked the auto, especialy for racing. Sure, in real life, if its a daily driver, and your not an auto enthusiast, and you just use your car to get from A to B, then its different. But usualy, real enthusiasts tend to like manual better, because its more fun, and you have a LOT more control over the car.