kutscherd Pretty much what ntozier said. Follow those instructions (although if you're using Linux, I'm not a fan of the instructions to edit /etc/crontab and I'm too lazy to create a Github account to submit an edit that may end up getting rejected anyway ðī ) and any tickets sent to the mailbox set up in Admin -> Emails -> Emails which has "Fetching Email via IMAP or POP" set to enabled will automatically get converted into a ticket. This is also where you define which department the tickets will default to.
If you're having problems, let us know what version of osTicket you're using, server environment it's running on, mailbox setup (don't post passwords or account details obviously!) and the problems encountered and somebody will hopefully have the answer. Who knows, maybe even I'll know ðĪŠ