How do I get the flippers and How do I get in the gamebuilding Where..

To get into the building is simple. Eventually throughout the game, it will open. It's simply still closed until then. There's nothing important in there anyways.

To get the flippers, a Minish by the lake will tell you about the library. I don't think you need to find him though... So go to the library, go to the second floor, and outside from there. Turn small and head back inside, and go talk to the Minish near the old man. (There's a ledge on one of the bookcases where you can get up) After you're done, go turn normal size, and talk to the librarian on the left. She'll tell you about finding checked out books. First is a girl with a cat, then a professor who's house you can now get into, and last the mayor. After getting the books back for her, you can climb the bookcase to the Elder, who has the flippers hidden in a chamber he'll send you to.