Disney has thankfully simplified the ticketing process as compared to the covid years and the time that followed. You may have heard of park reservations, but if you have a regular single day or multiday ticket, you do not need a reservation. You can get your tickets here.
The biggest decision you'll need to make when purchasing your tickets is whether or not to add the Park Hopper option, which allows you to visit more than one park in a day. If you have this option, you'll be able to start at one park and then change to another, subject to availability. You can go to as many parks in a day as you'd like. Check Disney's site for more information.
I'm going to make this simple: deeply discounted Disney tickets are simply not offered to the public anymore, and if you see anyone purporting to offer them, they are either fraudulent, or you are going to have to sit through a timeshare presentation or something of the sort that is going to eat up all of your time. For ease of administration, I recommend just buying them direct from Disney, though you can usually find (very) modestly discounted tickets from Undercover Tourist.