When you first start playing Fallout 76 Wastelanders, you will quickly come to a side quest that asks you to make some armor. You will also need that armor, as enemies hit surprisingly hard at the start of the game. You will need a couple of different resources to build it, one of which is leather.
To get your hands on some leather at this point in the game, the best this to do is take down some Brahmin. Brahmin are the mutated, two-headed cows that you can find as you explore the world. There are three Brahim located in the town of Flatwoods, near the river.
You will need to venture into Flatwoods to find out some information about the Overseer, so it makes sense to grab some resources while you are on the move. You can see the exact location where you can find the Brahmin marked on the map above.
This group of Brahmin should respawn a few hours after you kill them, but you should be able to get all the leather you need to make your armor from the first group.
Just whack them with a melee weapon. Brahmin take forever to become aggressive, and even then, they either just run away or continue to stand there. Gather all the resources they drop when they die, then move on. When you get back to the armor workbench, you select the option to scrap all your junk, and you will make some leather.
As you get further into the game, you can get leather from other sources. Some lootable items will give you plenty of leather when you break them down, such as teddy bears. You can also get leather from other animal hides as well, so whenever you see a creature in the wild, be sure to give it a few good smacks with a melee weapon, then harvest the corpse for resources.