I don't remember exactly how I got started, probably by trying to replicate what bigger kids were doing with "intros" and demos. I don't know what would nowadays be a simple enough starting point. But I remember William Stallings' and Andrew Tannebaum's books on operating systems and computer architecture in the university 20 years ago. I don't know if they have been updated, but they might be good starting point, if you need to start from the beginning.
On the other hand, if you know basics of how to CPU works, what stack is, know binary and hexadecimal, and some C, you might be able to just read Dennis' book, just ignore things you don't get at first, since many things get explained later in the book, after which you can re-read sections you didn't quite understand first.
I don't treat anyone like this but I thought maybe they take it seriously if it's written like this.( not justify it.. ) I don't think anyone at Paypal offend from this. I didn't mean it while writing actually. If it's too much, forgive me for not being native english speaker. Anyway, not allowing to paste seems not ok for a giant tech company Paypal. In my country banks have similar stupid forms and ways to "protect" people I don't think it's helping but to day I didn't complain any of them. Because I know almost nobody take it contact forms like this seriously. For this case I wanted to make it interesting, maybe it'll get to somebody there.