Join Date: Jun 2004
Location: Northern Nevada
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The year doesn't actually matter--it's fine if you want to know out of curiosity, but knowing won't make it easier or harder to find parts.
As estaban said, you can measure the frame yourself, from the center of the bottom bracket (the place where the pedal crankarms attach, at the bottom of the frame) to the top of the seat tube. A 14.5- or 15-inch bike is really tiny--my wife is 5'1", and she rides a 17--so if it's that size, I'd be surprised if it's too big for you.
I'm not sure what you mean by "plastic cover on the outside of the sprocket"--is it the plastic guard between your right foot and the chainring? That isn't necessary, and none of my bikes have one, so I'm not sure how they're attached. If you want to replace it and you're lucky, you'll be able to unscrew the bolts (probably with an allen wrench, which you'll have to buy for a couple of dollars) and just bolt on a new one. If you're unlucky, it will be fastened in some intricate way involving removal of the crankarm or pedal, but you should be able to tell by looking at it carefully. Really, though, it's OK to ride without it.
Here's a useful tip: Take the bike to a bike shop (a real one, not a department store) and tell them what you've told us. If you ask them for a tune-up or something, of course you'll have to pay for that. But if you tell them you're a new cyclist and you'd like a tire patch kit, a spare tube and a set of tire levers (all of which you'll need eventually anyway, along with a pump), they'll show you how to pick them out. Then you're officially a customer, and you can say, "You know, I'd like to replace this plastic thing, too. Can you get me one of those?" They'll either say yes and give you a price or tell you it's unnecessary, and when it comes in, you can say, "Do I just unscrew these bolts to put it on?"
Of course if they do any work for you, you should expect to pay. But if you're a customer buying parts, it's perfectly acceptable to ask for advice.