Hey there, Samantha! I am so glad you found planDisney to ask your question!
You are one smart cookie, purchasing your parking before your trip. This is what I have done in the past as well. It just makes for a nice, smooth day.
When you arrive to the Disneyland Resort, you will want to head to either the
Mickey & Friends Parking Structure or the Pixar Pals Parking Structure. Your parking voucher will be accepted at either of these locations. Should you decide that you would like to upgrade to preferred parking on the day you visit, you would simply pay the difference at the parking booth.
Keep in mind that the parking structures are open an hour before the earliest Disneyland park opening, and it is a bit of a walk to get to the parks. The parking lot trams are not currently in service.
I hope this helps with planning your upcoming visit to Disneyland! Please come back and see us again if you have other questions.
Happy Traveling,
Tracy