In career mode, you start with a 125cc moto. Level up so you can advance to the 250cc class. Keep leveling up and eventually you will be able to join a Moto GP class team.
Hire engineers en keep ugrading your moto. Do not change to another moto or you lose your progress.
For the last upgrade you need a level 30 engineer. Those are available when you reach level 30.
The The G.O.A.T. achievement in MotoGP 09/10 worth 345 pointsObtain a reputation level of 30 in career mode to become the greatest of all time.