"how does one know what key a song is in?" - for instance. Well, if you learn a piece of music for real, do this a couple of times, you have 'in key' within the context of a song. Absent any such experience, "a complete beginner"? I don't know what to tell you, I have nothing to grab hold of. I don't know what kind of ear you have. One can say "look for the 'home' or 'at rest' chord first, the "One" chord and that should indicate the key... " but music is so contextual. We don't know you; some people are naturals and some are quite ill-suited for "music theory" work. What's your goal, what about it seems so necessary?
For instance I had been fooling around with music for a few yrs, written songs, become a kind of happening lead guitarist and arranged for a singer for demos for a label, and then led a prog rock band before I took any 'music theory' course. The first reason for that was I wanted to understand JS Bach and I knew I did not have the tools to. Now, the basics I picked up through reading music, and when I wasn't sure exactly how to accurately write I was doing on a score, I went to girls in school that had, you know, piano lessons and that.
So, I don't know what 'basic theory', or quite what 'theory' means to you. Some people never care about it particularly and get to be great at music.