How do you get the subtopic before the domain name

For example if i have a website called helpme.com and I wanted to create a forum or anything really... how would i get the link to be forums.helpme.com or if it was a service or anything, how would i do the name before the dotcom

forum.helpme.com
service.helpme.com

how does one do that with their site. Do i set up a folder on the host and just name it forum?

Please if anyone knows?
Thanks

This is a task for your webmaster (the person that runs the web server you're on) - not something that can happen via coding pages for the client. Google 'subdomain setup {webserver} help' (where {webserver} is Apache or IIS, whatever you're running) if you're running your own web server. You might get help there.