Each park has its own lot and parking is easy. You will pay a $6 fee at the toll plaza, then the parking CMs will direct you to a parking spot. If a tram isn't already waiting for you, one will be along within minutes. The parking situation at MK is different from all other parks because the MK parking lot is actually at the TTC. Once parked you are still a long way from the park. The tram will take you to the TTC plaza where you board a monorail or ferryboat for the rest of the trip to MK.
The best advice about parking is to write down exactly where you are parked! The lots are huge, and it's embarassing to be out there wandering around looking for your car.
Parking at the resorts is not permitted, except for dining or short visits to the resort.