Assuming New Horizons is similar to New Leaf for some things...
Make sure to get the following items:
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A swimsuit
So you can swim in the water. I played early-game New Leaf without one and couldn't swim :/
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A trash can
If there are perfect fruit trees, when they die, you'll get a rotten apple. The trash can will be used to dispose of 'em.
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Have at least two of each "tool" item
I heard shops stay open at night so idk if this is needed anymore, but if the tool breaks and shops are closed, you're out of luck. Again, it might be different in New Horizons.
As I've been hearing New Horizons will have crafting, maybe you should save materials you collect instead of selling them all because you may need it to craft something. I'm not sure how the crafting system works yet, but that's how it goes for crafting games I've played. It could be hell to search for an item to craft X when you sold it.
If you want a specific appearance in the game, you'll be able to use QR codes to get the clothing of what people designed. From what I've heard, QR codes from New Leaf will work in New Horizons, and there are many QR codes available on the Internet.
For example:
View attachment 20134
So go ahead and search for specific clothing from anime characters or whatever to find 'em and use in New Horizons. If it's a popular show or anime, there's a high chance the QR code for the clothes is available on the net.