I have an old home with 2 wire wiring (black/white). I have a 2-pole, 3 wire 20 amp-250 volt outlet. How do I wire this outlet? Copper screws on both sides and a green screw on top.

If I understand you correctly, you have a double pole rec. Probably for an air conditioner or something right? If so all you do is put the white on one screw and the black on the other screw. I don't know your home, but if it's the old style wiring you should have metal outlet boxes that are being used as the grounds for the outlets. If so you shouldn't need anything for the green screw. If it'll make you feel safer and if you have metal boxes, you can take a piece of bare copper wire and attach it to the box by screwing it to the box and the attach the green screw on the outlet.

First, outlets do not have poles, only switches have poles. Sounds like you have the incorrect outlet. What are you trying to do?

If you install that receptacle in place of a "standard" 120v receptacle you will burn up whatever it is you plug in.

You must run a new circuit to this new receptacle.

If the device you intend to use the plug for requires 220/240 volts and existing receptacle
Is meant for 110/120, the device will run at half capacity and most likely ruin it. If that's the case, you'll need to run a 12 gauge 2 conductor wire w/ ground to the panel from that location. The black and white wires will both be "hot" and the ground is just that. The 2 wires must be run to separate circuits (separate fuses/circuit breakers). Put some black tape around the end of white wire so the next poor slob who may work on it realizes it too is hot. Having said all that, it's all under the assumption your house offers 220 service. There are still some very old homes out there that still only have a 110 service box.

After sending my reply, I realize I should point out when I say to go to separate circuit breakers, they must also be on separate phases (or hot bars). If it's a newer panel one breaker above the other should accomplish this. Sorry.