Aeropress recommends using fine drip or espresso grounds when brewing your coffee for the best results. With these fine grinds, you'll get a more decadent coffee.
The best thing to do is buy coffee beans and a coffee grinder that can make these fine grounds. However, if you don't have a coffee grinder, you can also use pre-ground drip coffee, but you may need to use more coffee or allow it to steep for longer so it has the same strength as finer ground coffee beans.