Before filling a pressure tank with water does the pump need to be plugged in, then turn the water on?

If you're asking about a bladder tank and not a galvanized pressure tank the correct way to start is to turn the pump off and open a cold water faucet to bleed off the pressure. You the pump air into the tank at 2 PSI UNDER the cut in point on the control box. If your pump cuts in at 20 PSI you fill the tank with 18 pounds of air. Good luck, Tom

Assuming there is a proper operating foot valve, you should have water in the supply pipe equal to the water level in the well. Opening the valve will not let the water back out. Turn on the pump and watch the pressure gauge. You will probably have to open up a faucet or two to bleed out air in the pipes. Once the pump shuts off, check the house pressure and adjust the control valve as necessary.

before filling a pressure tank with water does the pump need to be plugged in, then turn the water on?

You won't get any water until you plug in the pump. Turn on water AFTER the pump turns off, or it might take a while for the pump to fill the tank.

You would want to note the pressure at which the pump turns off. Check the pressure gauge. Should be one either on the pump or near the pressure tank. You will probably want the pump to cut off in the 40 to 50 pound range of pressure.