I'm not quite sure what you're asking, but an AdWord ad isn't intended to appear on your webpage. Rather, it's designed to appear on the search pages when someone punches in the keyword/phrase you've chosen.
At the same time, if you're talking AdSense as opposed to an AdWord campaign, then any ad you want to appear on your page is, to some extent, dependent on Google's willingness to see your page as one that's relevant for the ad -- which, in any case, you won't have created yourself.
Duncan
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