This is going to depend on what webserver you are running, which you do not mention. I'll give you the answer in general terms which should be good enough for you to google how to configure in your specific application.
What you want to do is setup a VirtualHost with your primary domain and a few alternate domains. I'll give an example config below for Nginx as that is my webserver of choice.
listen 80 default_server;
listen [::]:80 default_server ipv6only=on;
index index.html index.htm;
server_name mydomain.com myotherdomain.com moredomains.net;
The important bit to look at there is the "server_name" portion where I've put multiple domain names. Each of those domains, if their DNS is setup properly, when visited would show the same content.
In apache, this might look something like this:
Hope that helps you get this solved!