For Cloud, see https://www.atlassian.com/software/jira/guides/getting-started/basics
For Server, see https://confluence.atlassian.com/jiracoreserver/jira-core-server-8-1-documentation-938846148.html (but check the version selected at the top right)
In both cases bear in mind the getting started docs I've linked to are not the only ones; Your "Jira" could be one, two or all three of the Jira applications - Jira Core, Jira Software and/or Jira Service Desk
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