Hi Michelle,
Fantasmic is a great fireworks show held on certain nights at Disney's Hollywood Studios, it is held in a huge amphitheatre which holds around 10,000 people but these seats fill up quickly as it's such a popular show. To get a good seat I would advise you to arrive at the amphitheatre at least an hour before show time, take some playing cards, books, snacks etc with you to help pass the time. Or you could book the Fantasmic Dinner Package, this is held in select restaurants, details
HERE. This gives you access to a special reserved section of the theatre and means you don't have to arrive so early to claim your seats, about 30 minutes before show time would be fine.
Luckily no seat in the amphitheatre is bad but my favourite view is right in the centre close to the top. When we've used the Fantasmic dinner package we like to sit as close to the dividing rope as possible and again towards the top of the theatre.
Have a great trip and be sure to check what dates Fantasmic is showing once the park hours for your stay have been released,
Jo