Reach A-Spec Level 40.

Getting to level 40 in A-Spec will require a lot of grinding. Until you get to LV 21, you should get the most you can out of Special Events and Licenses. You can only get experience for them once for each medal value, but at least Special Events give you a lot of it. From LV 21 you can start following the grinding guide in this thread. At LV 25 you should do Endurance Races to level up faster.