In addition to the previously-mentioned Youtube and Madden, I'll recommend a book, Take Your Eye Off the Ball. It's excellent at explaining the general responsibilities of every player on the field, and what teams look for when drafting them.
Take Your Eye Off the Ball 2.0 - Pat Kirwan
That’s a great one.
Love Movin' The Chains on SiriusXM. Here's the 2nd Edition of Pat's excellent book:
Take Your Eye Off the Ball 2.0: How to Watch Football by Knowing Where to Look
Goes into detail about the game both on and off the field. The book comes with a DVD as well.
There's a good book called Take Your Eye Off the Ball: How to Watch Football by Knowing Where to Look . I read the first edition, but I link the latest one I could find. It's the book I recommend to people if they want to learn about the nuts and bolts of football. And it's been my experience, the more you know about the sport you're watching the better the overall experience is.
Edit: Thank you for the gold anonymous redditor
I would check out the slight more up to date version.
I've been reading Take Your Eye Off the Ball . It's definitely NFL-focused, but really gives a lot of insight into the things going on that we don't normally look at during the plays--specifically O-Line and D-Line gamesmanship and QBs reading coverage and pressure pre-snap. I'm only about 1/3 of the way through it, but I'd recommend it so far.