Welcome to planDisney, Marta! I'm so glad you stopped by with your question and to hear you're planning a visit to the Happiest Place on Earth in just a few weeks. How exciting!
My suggestion for where to purchase your
theme park ticket would be on the
Disneyland Resort website itself! This way, during the purchase process, you'll be presented with the opportunity to add on the
Park Hopper and
Disney Genie+ options (more on those below) and you'll be able to make your
theme park reservation during the purchase process.
Since you're only planning to visit for one day, I would highly recommend adding on the Park Hopper option to your ticket. The Disneyland Resort has two parks -
Disneyland Park (the iconic spot where it all started) and
Disney California Adventure Park (a short walk across the esplanade). In my opinion, both parks are must-dos and have some unbelievable and unique attractions that you won't find anywhere else. With the Park Hopper option, you'll be able to start your day in one park and then hop between both parks - starting at 11:00 AM - until they close for the day.
Another feature I highly recommend is the Disney Genie+ Service. This comes packed with convenient features, but the most helpful part for your day trip will be getting access to
Lightning Lane entrances for select attractions. These queues are typically shorter, so you'll be able to fit more attractions into your day!
Thanks again for your question, Marta! I hope you have a fantastic time visiting the Disneyland Resort.
Have a Magical Day,
Greg