It's a common misconception among people with programming backgrounds that having prior knowledge and/or experience with modern programming languages and methods will be a great deal of help in learning ("retro") video game console assembly languages.
The logic is essentially the same, but the similarities pretty much end there. Theoretically, a person with zero programming experience but a couple of college level logic classes under his/her belt would have just as easy a time (consider me exhibit A).
Consider - when a typical modern executable runs, it's constantly referencing .DAT files, image files, whateverthehell else. There's one binary running, but it is constantly drawing data from a storehouse pof neatly organized files, the missing of any of which would render the program unrunnable. In a ROM, all of those extra files are packed right in, and usually just wedged into the ROM wherever they happen to fit, not in neatly separated compartments of the file. So you could have 1255 bytes of assembly language immediately followed by 2001 bytes of image data, followed by some more assembly that breaks in the middle for a table of character stat data... Whatever. It would be nearly impossible to simply look at a NES ROM in a hex editor and understand everything you're seeing.
To make matters worse, Nintendo purposely made the methods by which ROMs manipulate NES hardware extremely cryptic in order to make unlicensed game development difficult.
I personally never studied NES assembly (my medium is SNES), but I can tell ypou that what you'll want to use to get a started would be:
1) a file that lists all NES assembly commands
2) some type of resource that explains how these commands work together to make a game happen
3) a debugging emulator, so you can see all this stuff happen live in a game as it's running (this is probably THE most important thing, actually - with enough time and motivation, one could learn everything just by fiddling around with one of these)
4) ideally, experienced people who can help you figure it all out. Unfortunately, breaking in to the hobby can be tough sometimes, because gamer nerds tend to be a bit elitist, and a lot of people won't see the point in helping but will be all too quick to discourage and insult you. Sad but true. From what I hear, the folks at nintendoage.com are an exception to this - I've heard only good about how helpful they can be - but I think they tend to focus primarily on homebrew games, not hacks.
So, that's all I've got for ya. Learning all this isn't easy, but it's certainly not impossible. Just... Abandon your assumptions about how it all works first.
also, the only thing using a tbl in your hex editor will ever help you do, really, is manipulate the text in the game