I read that users of wordpress.com cannot add their own ads. WordPress.com has their own ads. I am not sure. If that is the case, this post may not be useful. But if you have your own hosting site, then your in business. This is pretty simple.
First, get a Google Adsense account. If you already have an account go to the next paragraph. If you don’t, go to Google AdSense and apply. After you submit your application, Google is going to review your application. As part of the application process, you’ll need to add code to your web site. Pick an ad size of 120×240 so that it can fit in your page. At first, the code will display nothing, but you need to keep the code on your web site during the duration of the application process. Once the review is complete, you’re going to see ads.
Once you have code, go to Appearance->Widgets. Look for the text widget in the Available Widgets section. Now the next step is going to depend on your theme. On my theme, drag to the text widget to the Secondary Widget Area. This will add a text widget to the Secondary Widget Area which represents the right side bar of the web page. Click on the Text widget that you just dragged to the Secondary Widget Area. The widget should expand to a title text box and an another text box, a bigger box, in the bottom.
Leave the title box blank. In the bottom box, add your code. Click on Save.
And that’s it.
Once you have one ad going, you can put two more ads for a maximum three per page. Or, one 300×600 ad unit. Basically, one 300×600 unit or three other ad units. You shouldn’t add one 300×600 ad unit and one more ad unit. The rule is here. http://support.google.com/adsense/answer/1346295?hl=en-GB#Google_ad_limit_per_page .