Um, start by clicking service desk in the menu? If it's not there, then you need to install it... For setting up users and so-on, start with https://confluence.atlassian.com/display/JIRA/Managing+Project+Permissions Service Desk does change the way that works for Service Desk projects, but you need to understand the basics of permissions before you can worry about service desk.
Meet the new and improved IT service management template for Jira Service Desk! Here are a few of the powerful features ready and waiting for you: Better focus on ITSM - clear ticket categories...
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