bigginer

Im new to building flight.so I don't understand how to pick the batteries. Batteries had 3 classifications like mah,c ratings, cells. So i pick the Emax mt 2213 brushless motor with 1045 propeller, and also picked the BLHeli 30A 4S ESC F16740, Now I want to get battery. I have 2s 800mah 20C...