What should I know before buying a bike (New & used) [10 points]?

I need a bike. I attend a college that has a good sized campus. I want to get a bike to make it easier/more fun to get around campus. The only problem is that I know NOTHING about bikes other than how to ride one. Oh I also know that apparently a hybrid bike is supposed to be best for general use (which is what i'll be using this bike for) but I could be wrong. Anyways is it worth buying a brand new one since I wont be using it like the pros do (not sure how pros use a bike but I'm its more often and more tedious than I would) or should i just get a used one of craigslist/ebay/amazon or whatever. Also what should I look for in a used or new bike so that i don't get cheated. (Just an of the top question WTF is 'fixed gear' and do I need that?)
 
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