If you're just slicing it up and tossing it into a stir fry (or whatever) in quarter sized "rounds" you can get away with not peeling it as most people wouldn't eat a disc of ginger that big - but it's pretty easy to peel, so I tend to peel it all the time.
TC, it should be in a big pile right next to the garlic and onions. Look for a firm, relatively smooth piece. Joints don't matter, though it's easier to peel a larger phalanx. :p If it looks wrinkled or weathered, it's old - you don't want it. Also, if you can't find a small enough piece, just snap off what you want.
<If you're gonna be dumb, you gotta be tough>
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