Definitely playing video games and fantasy (both already mentioned). Games will teach you about the language and forms of the offensive and defensive schemes as

Definitely playing video games and fantasy (both already mentioned). Games will teach you about the language and forms of the offensive and defensive schemes as well as the basic rules (why a penalty is awarded and what its repercussions are) and fantasy will keep you up to date with the the current players, which is nice because players rise, peak and fall very quickly in the NFL (for very sad reasons, but while it's here and on TV, I'll be watching it).