If you're talking about the Fantasmic Dessert package, then I can help a little, but I haven't personally experienced it. You book 60 days in advance (I think) and ask for the Fantasmic Dessert Package or something like that. You receive guaranteed seats on a balcony and a tray with some small desserts. I believe the cost is somewhere around $55 or so.
If you're talking about just getting regular seats, the best seats are on the ground right in front of the Rivers of America. No cost, I recommend showing up 60+ minutes early for good seats in the slow to medium season and 90 for busy seasons. Usually there are two shows (9 and 10:30) and the second is almost always less crowded than the first.