If you're going to go online with Project Gotham Racing 3 on XBox 360 Live, here are some helpful hints to get you racing.
First, using your profile that is XBox 360 Live enabled, practice up a ton in the single player game. Unlock some great cars. All of the cars that you own in your local game are what will be available in your online game. You'll find a lot of super fast cars being raced by other players online. If all you have in your own garage is a very slow car, you're going to get frustrated.
Get a microphone headset. A lot of the players talk to each other while they race, and many are from countries different than your own. It's a lot of fun to chat with players from around the world as you zoom around the track.
When you choose a race and a class of car (say "E"), the system will put you into a game with other racers in the same class car and of the same general skill level - based on your online rating. That means everybody starts as a newbie, but as you play more you will start to meet more and more experienced racers.
Your lobby screen will show a little speaker symbol when you talk, and will show speaker symbols for any other talkers. There'll be a word rating of a person's level, as well as a # indicating their relative ranking in the online world. You also see a little color bar on the left which indicates the person's car's color.
Just as in real life, some racers will be friendly and a few will be jerks whose sole intention is to spin you out. Just stay away from the spinners and have fun with the racers. It's a blast!