The sizes of Screwdriver you use most often on a heli are usually 1.5, 2.0, 2.5, & 3.0 mm. The best advice here is to start with a decent quality basic set like Tarot Hex Screwdriver.
On RC helicopters, you mostly find Phillips Heads, but you may come across flat head, and very rarely Torx. For the small metric sockets, The sizes most commonly found on RC helicopters (RC in general actually) are 4.0B, 4.5B, 5.5B & 7.0B. More Screwdrivers will be available in Tools Kits.