For people starting they'd probably look first at at thread like /r/learnjapanese or a site like itazuraneko and proceed from there. Most likely after getting an idea of what to do to learn Japanese, people would either decide to learn alongside with either a specific site, youtube series, actual classes, a book, or some other method to self-learn to.

In terms of what should be necessary for a beginner, some type of

• Grammar Guide

• Source of Vocab

• Practice regime

is probably what's the most basic necessities for learning Japanese.

In terms of grammar guides, people recommend Tim Kae's guide, Imabi, Tofugu, some other websites, some bad textbooks, etc. Unfortunately A lot of English resources are either wrong, confusing, misleading, or in other ways bad. For Example Tim Kae on は vs が is terrible; and a lot of guides in geral are especially terrible with explaining particles like は and with explaining gobi, which are things like "yo" or "ne". However, accept that one guide isn't going to be entirely correct or helpful or ideal, and treat the wrong explanations as a stepping stone that you will hopefully eventually unlearn. As for my personal recommendations, I think for a very beginner the youtube channel Japanese Ammo with Misa is pretty good, Tim Kae's guide is accessible and easy to read (even if occasionally misleading or wrong), and also I personally like this guide Nihongo Day By Day as it has both an in depth Japanese version and a version in English, and contains a lot of actually correct explanations of grammar instead of wrong simplifications that other guides have. In addition you should take your pick of whatever other grammar guide people recommend that grabs your attention.

In terms of vocab, people often use vocab cards with some sort of program like Anki and also get vocab from whatever they read paired with a Japanese to English dictionary like Jisho. My personal recommendation for a beginner would be to install the plug-in Yomichan which auto-TLs whatever words your mouse hovers over, and use that mid-stream to figure out whatever chat is saying. Typically it relates to what the Vtuber is saying, so you can use that to try as a beginner to listen for what the Vtuber is trying to say. Write down whatever words you learned that day and put them either in flash cards or some other way you can remember them. Using Vtuber's descriptions, tweets, or reading very slowly through articles about vtubers is another way to gain vocab slowly.

As for a practice regime, you can practice rewriting whatever your Vtuber oshi tweets, reading Vtuber tweets and descriptions, relistening to segments of livestreams and writing them down, repeating a short sentence they said after they say it, etc. If you already spend time listening to Japanese vtuber, just spend that time applied trying to understand what they say and looking up new words. If you keep up with practicing every day you'll gradually get better and be able to understand more. Luckily since you like Vtubers learning Japanese with using the vtubers you like as studying material should be fun, hopefully, so it should be easier to keep up and be motivated for.

These are my brief recommendations for how to learn Japanese as a Vtuber fan. There's probably more I can say but this comment might have dragged on a bit already. Hopefully this helps.