The theory of website creation... whoof. That's a tall order.
I apologize in advance if this comes across as too simple, but it's what I used to explain it to my cousins: At it's heart, a webpage is special document sitting on a special computer that, when read from your browser, displays its content. That's really it. In order to get the document to the computer (aka your server), you use a file copy program, like FTP, or you edit the file directly on the server.
Now how you do all that depends on what app you use, which you also asked about The most simple 'app' is to just use basic HTML, however it requires you actually knowing HTML, which a lot of people don't. That's why people use apps like WordPress. While I do adore WordPress, the problem with it (and all apps like it) is they give you too many options and make it hard for the very new to see what they want to do.
You can use any app you feel comfortable with, and I'd actually suggest installing a couple of them in their own folders (you can do this via our one-click installer) and play with them. When you find the 'right' one for you, you'll know it.