Hi mrudula1290, If you have outside plugins you’d like to install on our managed hosting service, you will need our Business plan, because outside plugins and themes add complexity to our support and maintenance. https://wordpress.com/business
Our service includes hosting, as well as SSL, direct WordPress support, maintenance, security, and many other features that tend to cost extra with other providers.
Or, you are welcome to use your own free copy of WordPress with another host, and use community resources or hired contractors as needed. Then you can install plugins, but you’ll be in charge of maintenance and support for your site: http://en.support.wordpress.com/com-vs-org/
Does that help? If not, send more details and we’ll go from there.