If you have a site you are running at present which doesnt have a license, then indeed you would purchase the self hosted platform as mentioned above. Once you have purchased
download the software and upload over the files you already have there, in order to ensure you have the correct valid files present
If you are any versions behind, run the upgrader from /admin/upgrade at this point, and it will ask for your license key
If you were already up to date, go to your admin CP, visit System>License key, and change the license key to the one you purchased.