Well, as you'd expect, as a programmer I highly advise you learn the 6502 assembly language. Learn about the NES, and all the limits that you have to work with. There really is no "easy way" to make a NES game. If you want a game to truly be how you want it to be, you have to make a game engine, which involves coming up with level/map formats and ways to store music and how you'll handle AI (and much more). This, of course, requires knowledge of a programming language.
Learning 6502 isn't the hardest thing in the world. Once you understand the basics (loading and storing, loops, subroutines, etc.) you have a lot to work with. After learning the basics you can study more in depth as you program. You can see exactly how everything works and have a better understanding. Though this comes with time.
If you're thinking of using Metroid as a ROM to hack and make Metroid 3, I'd say that you're going to find yourself rather stuck in a lot of places. Metroid I'm pretty sure only allows for 256 unique screens. I assume Super Metroid allows for a lot more. Also, it doesn't know how to handle a lot of the items you get in Super Metroid. You'd have to probably program in handling what exactly those items do.
Also, I was incredibly disappointed when I played Metroid one, because I'd played it
after I'd played a lot of Super Metroid. The fight with Kraid is ABYSMAL! You'd probably want Kraid to be about two screens tall like he is in Super Metroid. I'm pretty sure that's not happening with Metroid one's engine. If you could prove me wrong, great. But I suggest that you learn how to program, as I learned how to program for the NES completely from scratch, because you'll find yourself able to get a lot further.
For some starting out tutorials:
www.freewebs.com/the_bott/gbaguy.zip
I don't know if this guy's tutorials are around on his site anymore, so a while back I saved them and uploaded them onto my site. Anyways, lots of people here (as well as myself) think these tutorials aren't very accurate. I thought they were great for starting out though. They give you a good walk through learning the basics of 6502. That's what I started out with.