When starting out on your Animal Crossing New Horizons island you won't be able to access all areas. Some will be hidden away at the top of high ledges and cliffs, and you won't be able to get up ...
In Animal Crossing: New Horizons, you can use a ladder to climb cliffs. When you are against the cliff, hold the ladder, place it next to the cliff face, and then you can climb it to reach the top.