If you are looking to include ads on your blog, you can sign up for WordAds. (WordAds works on WordPress.com sites and self-hosted sites with the Jetpack plugin.) Learn more and sign up for WordAds.
It's worth noting that a site generally needs thousands of page views each month to earn meaningful advertising revenue. What you earn can vary greatly per site, depending on the following factors:
- The amount of traffic.
- Where the majority of the traffic is coming from. (North American ad placements currently pay-out at a much greater level than international ads.)
- How many are using ad blockers.
- The actual ads that are displayed.
You can find information about monetizing your site including some other options here: