Tools. A fishing rod if it's there the first day. That's always what I end up buying first. Then I can make more bells to pay off my house payment then make my character look like I want.
I'll definitely be buying a fishing rod (hopefully they'll have one in store on the first day!) and then I'll be able to do some mega-fishing and start saving those bells for my mortgage and furniture. Then I'll probably go buy some clothes with fish money Oh! and some new boots!