Get Super Metroid on VC. It'll run you like $8 and it's the best Metroid.
The only thing storywise you need to know is that Samus is a bounty hunter. There are a few major "badguys" like the Space Pirates, who always revive Ridley. Mother Brain controls the space pirates defenses. Metroids like to suck the life out of whatever they can get a hold of.
I'm sure someone can tell you the details, but they don't really matter. Sorta like knowing all the backstory of Zelda doesn't matter for any new game.








