First pick a domain name and register it. This is cheap - maybe $10 - $20 for a year or two.

Second, find a hosting company so you can host your domain. Cost varies.

Easiest way is to choose a hosting company first, then on their website, apply for your chosen name. Lets say it's global1.net. They will become your registrant. When you sign up for the domain name you will find out the price, which varies for the different kinds of domains but it quite cheap.

You can have a domain name without having it hosted anywhere but obviously can't do much with it. Probably best to Google hosting companies and have a look at their prices and conditions such as bandwidth etc.

A domain name is something like: mycompany.com so people can access your website at: http://mycompany.com

Domain names can be bought for < $10/year at sites like
http://namesilo.com
http://namecheap.com
http://porkbun.com

Then you need somewhere to host your website.
What type of website do you need. Do you just want to host a blog? Do you have a company? Do you need to sell things
via the Internet?

When I had a website I registered with one company and hosted with another, and transferring the domain from the registrar to the host was difficult. I would register my domain with the host you plan to use.