Google Adsense is the easiest to install on your site. You install some JavaScript in the sidebar or in a WordPress widget and it starts showing relevant ads; the page generally starts showing ads immediately (about 10 minutes), it takes a while for the ads on that page to be “relevant” to that page.
I don’t think they reject (m)any sites so that’s not an issue. You make a few cents for every 1,000 or so impressions, but where the real money is when people click on your ads; this can be pennies or dollars per click.
Warning! Do not click on your own ads! They will catch you and kick you out of the program (it’s against your contract with them).
I think it takes a little bit to get through the site to pick ad types and sizes but once you’re done it’s ready to go. Don’t get focused on color and size you can always change it later (just name everything with relevant names so you can change it later). No matter how much time you spend configuring in the Google AdSense area it’s still the just as easy to get it on your site; so get something on your site so you can see how it’s working.
I usually use the really tall ads or the really wide ads, they tend to have 3-4 text ads in one window or one graphic ad. I’ll generally put them in a page layout so they automatically appear on every page on the site.
You can make changes to your ads at any time via the Google interface but it takes a few minutes for the ad changes to propagate down. For many web browsers you need to open up a new window or the Javascript remembers the old settings.
