2 very common causes for chronic flats.
1. No rim tape... this is cheap thick tape that goes on inside of rim. Prevents spokes from digging into tube.
2. Presta valves not being closed. If your tires use presta, you have to screw in a little nut after you inflate them. (have to unscrew nut before you inflats) -- You'll know this is the problem if your tires go flat, but you just have to inflate them again for them to be full.