Before your appointment, here's a checklist
I recommend:

Car working: Your car should be in good working order with no check engine lights. While some sensors/check engine lights can be disabled, others are more critical to the electronic fuel injection system.

Fresh cap, wires, and rotor button: It's a good idea to have a fairly fresh cap, plug wires and rotor button, if applicable. Stock or NGK is usually best, MSD would be another option.

NGK spark plugs: Your car should have NGK spark plugs of the proper heat range. They should be gapped appropiately. NGK spark plugs are stocked at most auto parts stores. Mainstream Performance can provide them if you arrange prior to your appointment. I do not keep them in stock.