Yes, Burnout is right, In WordPress.com subdomain blog you canot edit metatags or adding ads are not possible.
you both mean that i buy my own top level domain for this purpose and king from where i find ad sense plugin at word press.
Top level domain is not required. You need a paid hosting or a free hosting which supports WordPress plugings.
First of all get your own domain and host, have you imagined if they close your site? After serch for plugind on wordpress site, there many of them to add adsense code.
I'll try to explain this a bit more clearer. The company Wordpress created the free PHP application called "Wordpress". On their website Wordpress.com, they offer the application to be used and hosted on their servers to paying customers. However, they do not allow those customers to have server access to modify the application itself. Only whatever is offered through the admin panel. But at Wordpress.org, Wordpress simply offers the application as a free download for anybody who would want to download the application and use it themselves instead of having it hosted on Wordpress.com with no server access. If you are going to download the application and use it, then you need a server to put it on. Your own computer can be setup as a server, however that is not recommended for many different reasons. It would be best to just try and find a web hosting company that you can upload the application to and then use it however you want on their services instead of being limited.
I've never used Wordpress.com myself, but I've downloaded the APP from Wordpress.org and put it on my web hosts servers. It is a bit more difficult to use. But not that bad even if you only know basic HTML. If you wanted to add meta tags into your site you can just login to your web hosts filemanager or use an FTP client to connect to your files and go into the wp-content folder, then the themes folder and then go into whatever theme you are using and then edit the header.php file and add or remove whatever meta tags you want. Within these same files you can also add your Adsense coding. It's just a matter of finding out where the best place to put it is. There may be a file called sidebar.php which will be the coding for your sidebar, you could put Adsense in there. Or there might be want for the footer, footer.php, etc.