Hello and welcome! Congratulations for looking for your answers now!
This might sound like a stupid question, but have you looked into fish health and what you'll need to know to maintain a pond? I think a lot of people jump into it head first, get their pond built, and things start going downhill from there. Learn about the nitrogen cycle, fish loads..... Algae is your friend.....
You said 8x8, but how deep are you planning?
Like
@sissy said, your liner will be your most important purchase -- EPDM is the best, but also most expensive. If you skimp on the liner, you're asking for trouble. Oh, and don't forget a good underlayment. You'll need a pump and a filter. There are too many types to say which is best. You'll need a pond water test kit. You
won't need a heater to keep the water warm in the winter, but if you live where the water freezes, you'll need something to keep a hole open in the ice. Also a good item is an aerator and air stone to provide oxygen.
Are you going to have a bottom drain? These require different equipment and set-up than a submersible pump. What about a skimmer? Not absolutely necessary, but they are helpful.