On this page of our guide to Animal Crossing New Horizons, we explain how you can get to the upper parts of your island.
The island where you live in Animal Crossing is not fully accessible from the beginning of the game. The first obstacle you will encounter are the rivers (how to cross them is explained here). The second constraint are the higher parts, which can be climbed only with the right tools.
The easiest way to get higher is by using a ladder. You get it during the game, so if you haven't got it yet, just keep playing - you will acquire it after some time. Specifically, it's the moment when you build Nook's Cranny store. Then ask Tom Nook what to do - he'll ask you to build a bridge and three apartments. After completing this task, Tom will invite you to his house. Then, he will give you a crafting recipe for the ladder.
At some point you will also be able to fund public projects. These include mostly various types of bridges or stairs. Thanks to them we don't have to use tools every time. Unfortunately, these are not cheap investments, so you will have to carry a ladder or vaulting pole for a long time. Both tools do not wear out or deteriorate, so you don't have to make new copies of them every now and then.