OK you must have got the domain name from some registrar, on your domain settings page, there must be DNS Settings, DNS Management or Advance DNS Controls.
If you have already assigned some values in NS(name Server) then remove them, use DNS management, you have to make changes in "A" records, Please note here, some Registrars have separate sections for every record TYPE (MX, A, CNAME etc etc) some have one table, where you can make selection from drop down, which TYPE of record you are entering. so first record to be entered is "A" record.
Change the IP of your A record to your Public IP
so taking your example
RECORD/POINTS TO: your public IP (example 184.108.40.206)
TTL : usually default is 14400 (in seconds) keep it that way unless you want lower value
Now create another CNAME record
RECORD/POINTS TO: moodlehelper.com (remember you have already assigned a host to moodlehelper.com above)
TTL : 14400
if you are making use of IP v6, then you'll be creating AAAA Records instead of A (but just for now, ignore IPv6 and keep yourself o IPv4)
also keep in mind, it will take some 5 minutes to 4 hours (theoretical update time is 48-72 hours) for your domain to point at your server.
Before actually pointing your domain name to your server, do check your server getting response from external internet, using SOME OTHER INTERNET connection, as it may not open up server if you are using the same internet as the one your server is on.
in 10-30 mins after records update, do ping your domain to see if it shows your server IP.