Q: What if I already have a domain name?

A: That is a good start!
All you need now is a good hosting company.

Why?
If you want a web site, you will need web hosting. Basically, web hosting provides a way for people to access your web site through the internet. Without it, there would be no way for people to see the page.

And again, to Understand Web Hosting, You Need To Know What a Web Page Is

A web page is, simply put, a file. When you or your designer builds a web page, you or they save it as a file on a computer.

So, in order to get the web page - a file - visible on the internet, you need to rent some space on a computer that specializes in serving files out across the internet to "host" your web page for you.