Just setup an account at http://adsense.google.com, create your ad. I try to make mine appear much the same as my page content. The interface doesn't generate code for you anymore as far as I can see, but it will show you an example of what you've created.

Follow the instructions on how to implement the ad. Ad positioning can have an impact on click through rate and they have some pages on how to optimize the ad's value. The ads also make a great filler in a spot that might look otherwise dead.

if your page layout allows, try to use a positioning scheme that allows you to put the ad call after all your content loads, if at all possible (pretty close to impossible with a table layout). Google adsense, adwords and analytics seem to be serving up slower and slower lately and f you can get your page content loaded while google is deciding to serve up your ad at least the page won't appear to load so damn slow...