Getting Tickets
Visiting Disney's parks now requires two things:
You need to have a park ticket
You need to have a reservation for the park you intend to visit.
While these are separate items, you should view them as part of a single process and do them at the same time.
Here's the process:
Check the park reservation calendar and make sure that there are reservations available for the park you want to visit on the days you want to go
Do not just buy tickets and show up at Disney and expect to get in. Reservations do run out, and there's a chance you'll be wasting your time and money. At a minimum, dealing with the issue at the park gate will waste valuable park time.
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 after 2 pm, subject to availability. You can go to as many parks in a day as you'd like, but you cannot begin hopping until 2 pm. 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.