Where to put the museum
Of course the answer is "wherever you want" because this is your island and you can do whatever you want with it. However, I'm a fairly experienced Animal Crossing player, so I might get some advice on where you want this building much later in the game. But keep one thing in mind, if you don't. your choice, you can always change it later, although unlocking takes time and costs quite a few bells.
With all of that out of the way, here are some ideas. First of all, do you like the Museum? Are you like me and frantically rushing in with the first new cash you catch? Then you probably want it to be centrally located. I put mine right next to resident services, like literally next door, so I can go there and handle all of my day-to-day chores, including dropping off new museum specimens. I had also already located my house just north of here, so it's one of the first buildings I can access every day.
If that's not really your thing, you have less to worry about, and you can be more selective about where the museum is located in Animal Crossing: New Horizons. Place it closer to the ocean so you can drop new fish, then move it around once you've completed this collection dozens of hours from now. Or just put it out of the way somewhere, so you don't have to face the huge building blocking your view.
Basically my advice for where to put the museum in Animal Crossing: New Horizons is to think about how much you use it. Place it near frequently traveled locations if there are a lot of them, or place it where it looks best if not.
