In your adsense account in google, you have the option to choose whether your ads will be base on your blog's content or not. When you choose the "base on blog's content" you have to put labels, tags or keywords in every post you've made. Because in that keywords, the google adsense will check and find relevant ads that is related to your latest content.
If you're looking for highest paying keywords in adsense, you can search it in my blogitrob search engine. Or if you are looking for a tool that can show you the data about the rate of your keyword then you should check this out:
Go to my blogitrob search engine, type "Keyword Estimator" wait for the site to be fully loaded, then a list of result will be shown, if you see the "Google Adwords: Traffic Estimator" click it. Then you'll be directed to its page. It has steps and guidelines provided so you don't have to worry on how you will use it. The tool will show how many people are interested on that topic or keyword, it will also show the rate, estimated ad position, estimated cost per click (day), estimated cost per click (a week).
You better check your keyword now! It will help you increase your earnings in adsense.