If you want to stick with WordPress.com but would like you blog to appear as a particular domain (like yourdomain.com) then you can do that right from within the backend. Head over to Store and pick the first package called Add a Domain:
Next you’ll be presented with a box where you can type in your desired domain name, including the extension (say something.com):
Once you click the Add Domain to Blog option, the system will check if it’s available or not. If it is, you’ll be prompted for payment details. I believe it’s US $17 in total, $5 for the domain and $12 for mapping it to your new WordPress.com address. The system does this automatically for you, however it will take up to 24 hours for the new domain to be available on your new site.