In Animal Crossing: New Horizons, you can chop down trees by using an axe. There are three types of axes available in the game: the Flimsy Axe, the Stone Axe, and the Golden Axe. Each type of axe has a different durability, so be sure to use the right one for the job.
To chop down a tree, first equip your axe and approach the tree. Then press and hold the A button to swing your axe and cut through the trunk. Once the trunk is severed, the tree will fall over and disappear from your island. Keep in mind that if you chop down a fruit-bearing tree, it will no longer produce fruit.