1. First you need to go to Editor>Manage Export Templates then click download. Then you export by going to Project>Export then click add and click a platform. I'll show how to do HTML5 and windows:
HTML5 export: Click export project at the bottom. After you download put the files (game_name.pck, game_name.js, game_name.wasm, game_name.audio.worklet.js, game_name.html, game_name.png, favicon.png) into a folder. Rename game_name.html into index.html, then zip the file and upload it to itch.
Windows export: In the settings check add pck then click export project. After you download put the files (game_name.exe, game_name.pck) put them into a folder and then zip it.
2. I didn't really use may tutorials when I learned Godot but I used the q&a and forums a lot which I would recommend.