The first step is choosing a plan according to the number of users you expect and the storage you need. Pay for it if it is not the Free Plan.
Then, just choose a sub-domain name along with one of the MooDIY domain choices, This will become your Moodle sitename and the base URL e.g. mymoodlename.moodiydomain.com .
That is all. Your Moodle site will be created right away and a link will be provided to you for logging into it as a Moodle admin.