hey i've got it, once you complete all of the races C - A class, you can then tune, but that is about all, keep in mind its only the Prologue (i believe its only half of whats in the full game)
I havent played in a few weeks, but when taking a corner, you need to keep on the accelerator, even just slightly, if not youll begin to lose traction and bang!
so ideally, slow down heaps as you get to the start of the corner and power through it.
hope this helps
Steve