Are you sure you need to add cooling capacity?
Is your new engine both broken in and well tuned?
If after this you still want to add an electric fan, you will need some relays and a knowledge of your under-hood electrics and wiring. There should be plenty of on-line sources for instructions in hooking one up.
I only used an electric fan in my 66 Charger, and it was wired so it only ran when the engine was running. This eliminated the electrical strain at start-up. Some people like to have it thermostatically controlled. I didn't.
So, answer the first two questions, then let's go from there.