Aside from basic knowledge of music theory, what would you say I should focus on as a beginning player. I'm guessing it's 'learning the scales' (major, minor) and the correct fingering of them.
Well, the way I see it, you have about 4 directions in which to proceed; formal lessons, informal lessons, combination of formal and informal, and play-by-ear. Some here use the term "self-taught", but in reality, we all learn something from others. So, I don't think there is a such thing as "completely self-taught".
As for me, I chose the "play-by-ear" semi-self-taught method. My approach is fraught with what the formalist call "bad habits"; some think my unique "by-ear" playing is appalling; others seem to like it.
Since one of my "appalling" music videos on YouTube has over 1.5 million views, I figure there may be a demand for "appalling" piano playing.
So, my advice to you in regards to learning to play the piano... decide which route, or combinations of routes you want to take.
Learning key scales and basic chord combinations is a great place to start, lessons or not.
My 13 year old granddaughter was taking formal piano lessons for about 3 years. Her teacher did not want her to attempt to play anything at all by ear; all by written music, and would scold her if she attempted to learn anything by ear. After 3 years of lessons, she lost interest and is no longer taking piano lessons. I was so sorry to hear that.
In my view, learning to play the piano should be fun, although it can be hard work. Never take the "fun-factor" out of piano playing.
Good luck, whatever you decide to do!