WordPress.com isn't a domain registrar so you can't transfer your domain to WordPress.com. You will need to manage the domain through your domain registrar.
However, you can map the domain to your WordPress.com site by changing your nameservers through your domain registrar's site in your control panel. You will need to update the name servers as below:
Then you will need to purchase our domain mapping upgrade in your Upgrades panel in your WordPress.com account.
For more information please see: