You cannot enter a park without a valid ticket and a reservation (currently up until September 26th 2021).
Step 1: Link Your Admission to Your Disney Account
To get started, you'll need valid park admission that’s linked to your Disney account.
Step 2: Create Your Party
When you begin to make a reservation, you’ll be prompted to create your party from your Family & Friends list.
Step 3: Select a Date and Park
Choose the date and the theme park that you’d like to visit from the available reservations.
Step 4: Review and Confirm Your Plans
Carefully review and confirm your selected park and date.
You'll have to keep in mind that you cannot book dining this far out, so you'll have to plan now what parks you want to visit on what days, and then sort out dining at a later date. Also, park hopping is not available, so you'll only be able to book one park each day.
So make sure your tickets are added to My Disney Experience, and each person has their own My Disney Experience account all hooked up together (using the friends and family feature) and make sure everyone's ticket matches up. Then when reservations open, you'll be able to choose which park you want to book for what day for your entire stay or ticket length.