Depends what kind of website you want to have :).
If you are looking for a full independent domain like: Mydomain.com you will have to register the mydomain.com (url) which usually costs around $10 for a year. And you will need a place online where the files on this site are hosted (put on) and this also costs a few dollars a month.
If you are looking for a totally free domain, then you will always register a name on a subdomain. (example: Mydomain.wordpress.com) you are registering a domain on wordpress.com. These are called Web 2.0's. Each web 2.0 has a different system to build domains.
Most famous ones:
http://strikingly.com (I like this one a lot)
hope this helpes :)
It actually depends on the type of your future website. But there are some website builders which are quite versatile, like Wix (my favorite) or WordPress. Here's more about them and some other web-builders as well, hope it helps. Take care!