Hey there Laila,
Welcome to planDisney and happy early birthday! I'm happy to hear you'll get to spend it in the Happiest Place on Earth with friends - what a great way to celebrate!
The price for
1-day theme park tickets depends on the day that you plan to visit the parks. As you can see in this
1-day ticket FAQ page, the busier days tend to need a higher ticket while less busy days require a lower tier ticket. Though I will note that ticket prices are subject to change at any time, from what I've seen over the years, these tiers are fixed for the calendar year so typically the price of a ticket would increase only if the Disneyland Resort changed their ticket prices across the board.
Along with a valid theme park ticket, you also need
theme park reservations for each day you plan to visit the parks. Availability for these park reservations are limited and in some scenarios will run out. Since you're planning on purchasing a Park Hopper, you should likely be fine but I'd recommend keeping an eye on the
Theme Park Reservation Availability Calendar to see what's available for the day you plan to visit.
Finally, since you're visiting for your birthday, make sure to pick up a
complimentary birthday button located at most checkout locations in shops around both parks! This is a great way to get extra birthday greetings during your visit.
Thanks again for your question, Laila! I hope you have a wonderful birthday visit.
Have a Magical Day,
Greg