ok, first go to a site like godaddy or namecheap and buy one, they are usually around ten dollars.
First make sure you have a dedicated ip as it will not work if your port is anything other than the default 25565
then, once you have registered a domain, click the button that says my account and go to manage domains. Once you are there, click the domain you want to use and it should come up with a modify domains page. Scroll down and click on all host records. Go to sub domain settings and type into the first box something like "play or mc", like other servers do. Then type your server ip into the second box, and after that select A (adress) for the third one and finally put 1800 in the last box. Scroll down to the bottom and click save changes. You're done! It should take up to an hour for your server ip to update.
I hope this helped
