The URL of how people will find you online. Can be in the form of any of the following, being HTTP, HTTPs, WWW or non-WWW.
Did you know you receive additional trust points from Google for using https rather than HTTP? The reason is that SSL is a secure connection, which is a verification process to setup and proves who you are. Google wants to serve trusted websites to its users, and this helps your chances.
In regards to www vs. non-www, my preference is to keep it short and sweet, dropping the www. This is done in Google’s Webmaster Tools for which this free training course will not go into. An additional tip is when creating anchor text, don’t ever use the www version if anchoring with your branded URL.
For URL registration, my favorite domain register has always been GoDaddy and can be found at https://www.godaddy.com.