How can I get servers on the Mac OS?

You're going to have to be more specific. EG: Are you wanting to install macOS server (now defunct) on your computer or do you want to install a 3rd-party server product instead. If the latter, which one as there are many.

On the other hand you may mean how to access a specific server or servers? In which case the same questions apply: what are you trying to do? Is this at work, home or on the move? Which service are you trying to access? Email, file, ftp, web etc.

No-one here can begin to answer your question until you provide more information.

What kind of server?

SMTP server?

IMAP server?

file server?

web server?

database server?

calendar server?

directory server?

time server?

print server?

network server?

DHCP server?

DNS server?

proxy server?

authenticaton server?

or any of the myriad of other 'servers' that are out there?

and who are you serving? local users on your network? anyone on the internet?

All are possible, but I doubt you need them all.