Buy the domain
1) Go to the domain's DNS. Change the A record (only the one field I guess) to the domains IP.
Then, you can (if you want) go to cloudflare and do cloudflare. You +add site on cloudflare, enter the domain, then click ok when it retrieves what it finds for that domain's name servers, and it says change to karl, and you go to godaddy to the dns for that domain and change the nameservers to those karl.
2) Go into cPanel (log into it the way you usually do). Go to +Addon domains. Type in the domain. It will create a new folder in the public.html.
Now it should work when you create an index.php file there and put hello on it.