It's 1/2" NPT
which means the inside diameter of the hose needs to be 1/2" on the inside and usually that means you will be buying a 1/2" ID 3/4" OD
(inside/outside diameter) hose. I got mine from Home Depot along with a 1/2" plastic reducer and plastic clamp.
My advice would be to take the pump into Home Depot or Lowes and tell them what you are using it for & the guys in the plumbing section should be able to show you every thing you need.
As for getting the water back in the tank you will need more then just the flare nozzle. A Directional U-Tube
would be the easiest way to go.