Hey there, Andrea!  Welcome back to planDisney.
Walt Disney World Resort is such a large place, with lots of pieces to the magical puzzle -- we know it can be confusing, and that is exactly why we are here!
The parking lot for 
Magic Kingdom Park is at the Transportation and Ticket Center.  Theme park parking lots open approximately 1 hour prior to first Guest admittance, so for Magic Kingdom Park the 
parking lot usually opens around 7:00 am on days when Magic Kingdom Park opens at 9:00.  The Transportation and Ticket Center is located on the opposite side of Seven Seas Lagoon from Magic Kingdom Park, so to reach the entry gates, hop aboard the iconic 
monorail, or choose 
ferry transportation.  At the bottom of the monorail link above, you'll see a map.  Click on the blue "Magic Kingdom Park" words to the left of the map, and select "Resort Monorail - Transportation and Ticket Center".  You'll be able to zoom in and out and scroll on the map to view the parking areas, monorail loop, and ferry dock.  The express monorail and ferry usually begin operations around 7:30 am when Magic Kingdom Park opens at 9:00.
I'm not sure what you meant by check-in.  If you need to pick up ticket cards or activate tickets, this can be done at the 
Guest Relations booth to the right side of the entry gate to Magic Kingdom Park.  If instead, you were referring to entering the park with your 
Disney Park Pass reservation, simply head to the entry gates.  All Guests may enter up to an hour prior to official 
park opening time to enjoy Main Street, U.S.A., and make their way toward 
Cinderella Castle for the welcome show, 
Let the Magic Begin.  Andrea, if you meant something else, please feel free to 
write back.
Be sure to check out the yummy 
50th Anniversary snacks during your visit.  You will also be arriving just in time to enjoy some of the 
limited-time Valentine treats.  Wishing you a magical visit!
Anne Marie