What should someone just starting on YouTube know?

select your niche or category, video games, food, nature and many more, now you need to start creating your content and decide what days you will upload your content, you also need to take in consideration the quality, how long the video will be, music and effect in the video, tags and if you have social media you can reditect them from your social media to your youtube channel, there are many things about the planing, you can organize first your self but once you have an idea of the direction you want to take you start making the content and upload it regularly also don't forget to interact with your followers, that's important.

Now you are competing with very smart people. There are lot of good creators coming on YouTube , so the journey will not be easy.

Well dont expect to be easy and have petience ,rome wasnt build in one day
You need to focus on the quality you will provide to people
If you succed in making people watch all the duration of your videos then the altgoritm will like you

I would say a beginner must try to sell his products or services by marketing them in his YouTube videos.

Monetization of the videos (YouTube money) will probably not be profitable enough and your channel can be destroyed by YouTube without any warning.