Here's a quick way: Equip AGRO at the garage, go into Arcade, and play the "Welcome to Hollywood" race with 1 Easiest AI and no traffic. If you drive well, you can get about 2 AGRO each race and the races are short.
If you wanna check your progress, you can do so in the garage. Also, don't let it fill up during the race for nothing. If you're only level 1, you can't store more than one use. So, as a rule of thumb, just use it as soon as you have it.