Someone told me that you can add widgets or apps so that let my free blog receive advertisement?
Someone is not telling you the correct information. Someone is also not awre that if you want to monetize an blog and are serious about making income from it your do not waste any time here. You hire a web host and you set up a WordPress.org install.
There is only one advertising program called WordAds at WordPress.com and it’s for blogs on their own domains. Aside from a blog being accepted into WordAds http://www.wordads.co/ 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. Paid content and/or sponsored content is not allowed.
On free hosted WordPress.com blog we can use a contact form and/or a PayPal button for sale of only products or services created or provided by ourselves.
There is no upgrade you can purchase from WordPress.com that changes any of that. If you do require an ecommerce site, advertising, affiliate links, paid or sponsored content 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/
Note that we Volunteers answering support questions on these technical support forums cannot assist with any WordAds issues. You must contact WordAds Staff directly.
This thread is tagged for closure as WordAds issues are not discussed on these technical support forums.