What components are needed for a 20MHz square wave clock?
If you have a crystal oscillator (they usually come in a 4-pin can), you just have to apply power (and ground) to the oscillator, and it will produce a 20 MHz square wave, which can be fed directly into the Z80's clock input.

If you have a plain crystal (usually in a 2-pin can), the other answers show possible oscillator circuits.