Hi Stephanie!
Great choice planning to see Fantasmic! And it is just that...FANTASTIC! It lasts about 25-30 minutes, and includes everything from lights to fireworks to movie clips to props to moving water floats, you name it!
You can try to come early and save a few seats, but the problem may be they continually ask guests to slide to one side of the bleachers to allow other guests coming to sit. Also, when it starts to fill up, it may be difficult to get to the area you may be sitting if its close to the front or in the middle of the bleacher row.
Our family did the Fantasmic Dinner Package, which is where you eat at one of the participating restaurants (Hollywood and Vine, Mama Melrose, or Hollywood Brown Derby) and get a pass for Priority Seating to the show. Priority Seating is a little less crowded and offers excellent viewing of the show. I also noticed family members slipping in and out up to the last few minutes before the show, since it was less congested. This may be the route you want to take so you don't have to wait in line as long or if family wants to come later.
Good luck and have a great time!