Well first you have to export the game first, you are given a choice to where to put the game files, you also can pick from windows/mac/linux. There is also a button that only uploads the files inside the game that were used but that causes problems with images and plugins so dont push that button unless you want to copy files of the original onto the final export game.
You need right click > go to "new" to make a zip file, i am assuming you running windows because Mac and Linux have different ways to zip files. Put the final export files that RPG Maker MV made into that zip file. dont worry about names of the files its ok to do nothing with it or name whatever you want with it.
Go to itch.io click on the down arrow that looks like this V next to your profile, click on 'upload new game'. Everything should be easy to understand and follow. Just name your game, upload the zip file, put it has the computer you let it run and add words. Click public and then publish for everyone to see it and play it.