This is the oficial google resource for learning how to manage your adwords PPC campaign
http://www.google.com/adwords/learningcenter/text/index.htmlEssentially though you need to have the landing page optimised for the keywords you are bid on in order for you to get a higher quality score so you pay less per click, you also need to have your adwords advert content matching the term you are bidding on (and your landing page) so you get a decent click through rate (also improving your quality score) and a decent conversion rate.
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Adwords PPC Management