Hello friend!
Welcome to planDisney! I'm so excited to help you plan your first trip to Disneyland Park! It's such a special place to visit as it was where Walt Disney himself was personally involved from concept to creation.
As you are from Japan the best way
Theme Park Reservations can be made is through the
travel agent you used to make your original ticket purchase. However, if you can access your My Disneyland account or download the My Disneyland mobile app you should be able to make your Theme Park Reservations that way as well. Theme Park Reservations can be made 120 days before you arrive and you are within that window so don't delay so that you can secure the Parks you want.
If you a fan of the history of Disneyland Resort then consider purchasing a tour. A great one is Walt's Main Street Story Tour which ends with a visit to Walt's apartment in Town Square above the Disneyland Fire Station. I had a chance to see it and it was such an amazing experience. My favorite part was looking out the window onto Main Street U.S.A. and imagining what Walt was thinking when he looked out onto the Park and arriving guests.
Your arrival may time beautifully with the arrival of
San Fransokyo Square at
Disney California Adventure Park. It is themed after the movie "Big Hero 6" and one the best huggers ever- Baymax. I hope you get a chance to enjoy it!
Have a great time and Happy Planning!
-Nimmy