When people first start putting Google Adsense code into their blogs, they are likely to get PSAs - Public Service Ads. These are adverts that earn you no money and only occur when Google can't find any real ads to show. Now, that space is much better spent doing something useful rather than stealing your visitors for free.
How do you turn them off?

Advanced: If you are using blogger and want to tinker, go to your template, find the adsense code and after google_ad_client = "pub-XXXX"; start a new line and copy:
google_alternate_color = "0000FF";
This will display a blue square. Ofcourse, your background may not be blue so copy the colour from google_color_bg = "000000"; and paste it where the colour is so that you now have, for example: google_alternate_color = "000000";.

Or Normal: Go to your Adsense account - Setup - fill in the design you want - go to Alternate ads or colours - choose fill space or url - paste new code. You can either show a solid colour or an alternative advert such as for an affiliate program such as Amazon. Make sure you are earning money from your site at all times. Money.

However, the best way is to have good content and keywords. That way Google Adsense will display the best relevant adverts for your website and give you the best revenue and return. I explained how sort of in my past posts.