Fruit trees are a core Animal Crossing: New Horizons mechanic that you want to get to grips with fairly quickly. You start the game with fruit on your home island, but you can grow more.
Whenever you have a new or old fruit in your inventory, you have the option to sell it, eat it, plant it to make more trees on your island. Before you can farm any of these trees, you must have access to a shovel. You can get one from the Blathers when they arrive on your islands to help house the museum. Once the museum opens, he gives you the recipe, which you can freely use to move forward.
Now find a place where you want to plant your new fruit tree. Once you've found your sweet spot, get out your shovel and dig a hole. For the next step, go to your inventory and select the fruit you want to plant there. You want to keep holding the shovel for this piece. With your shovel in hand, you plan the fruit tree and you have to water it and take care of it to make it grow.
Growing things in Animal Crossing is a timely process, so you need to exercise a bit of patience throughout this process. Don't get too carried away as there are plenty of things to do on your island, but make sure to water it every once in a while to make it grow faster.
