You need a domain name like my_domainname.com to get started. You can buy .[dot others].
There are many registrars. Just google* for 'register domain name'.

I use godaddy.com, so I'll briefly describe.

Get A Domain Name

Open an account with godaddy.com

Type in the name you wish to register in the 'availability' box and search.

Follow the instructions to pay.

When finished, clicked 'Domain names' and it will list your domains bought.

Select the one you just bought say mydomain.com, click this.

Look for the nameservers on your right.

It should show godaddy's default nameservers like 'ns1.parkserver.com'.

Remember:
There are only 2 objectives in godaddy.
(1) buy the domain name

(2) change the nameserver to example 'ns8.yourwebhostserver' (once we have bought a web host account, your web host company will give your their nameserver - and you just change it here in godaddy)

Some Tips

Look for the 'Quick Checkout'. (ignore others and just get your domain name)

Try one year first if in doubt. So change the period (it defaults to 2) to 1.

There is an option 'privacy' if you do not wish others to see your name/address on the internet. If this is very important, then just spend another about $9 to get the 'proxy' for one year.
Later when you learn and know more about web hosting, you can opt not to renew the proxy and disclose details instead.

Use a virtual credit card or debit card for payment. These cards usually have a spending ceiling or you can set/request one to minimise/mitigate online fraud exposure.

Control Over Your Account

Opening an account and buying a domain name yourself gives you absolute control. If you do not like your web host company, get a new host and just change the nameservers.

Of course you need to backup your files in your old host account and transfer them to your new host account so that your website runs on your new host account.

* google
Always use google to help find answers. Go to www.google.com and type in the keywords you like to find. Press enter.

Note: I do not get any anything for recommending godaddy. It's just an example.