You cannot a dd any sponsor banners without getting your blog suspended. There is no blogger initiated advertising allowed on free hosted WordPress.com blogs.
There is only one advertising program called WordAds at WordPress.com and it’s for blogs on their own domains. This is the homepage for the WordAds program: http://www.wordads.co/ There you will find the application page as well as FAQs.
Aside from a blog being accepted into WordAds, no blogger initiated advertising, retailing or reselling the work or products created or services provided by anyone other than yourself is allowed on free hosted WordPress.com blogs. E-commerce transactions via shopping carts and the like cannot be conducted on free blogs from and being free hosted by WordPress.com
If you do require an ecommerce site, advertising and/or affiliate links you can hire a web host and get a free software install from http://wordpress.org Refer to > http://support.wordpress.com/com-vs-org/