A great way for you to get started is to take some time to read TCrouch's article on Motorbikes. This article lists general principles of what to expect racing on a motorcycle and how different it is from racing in a car, among some other issues.
Tips for New Motorbike Riders
Get used to riding the bikes that are in the 125cc class first. They handle similar to the big boys of the main MotoGP class, but don't have as much horsepower. With the 125cc bikes you can get acclimated to how to control the bikes in general but don't have to be as careful using the throttle. Once you're comfortable riding the 125's, move up to racing with bikes from the 250cc class. And finally, the ones from the main MotoGP class.
Do a bunch of laps in Time Attack mode to get familiar with racing on the bikes as well as all the circuits. During the offseason many MotoGP teams in real life go to do their extensive bike tests at Jerez because of the good variety of corners present on that track which benefits their research, so maybe that's the circuit you may want to try out. Stick to racing with the default bike setup for now.
Are there any specific problems you have controlling the bikes in MotoGP 08?
Kruza