Well first, get JIRA Software, not JIRA Core, so you can have Agile boards.
After that, it depends a lot on the kind of organization you have, your users' level of familiarity with the tool and the pain points in your workflow. For instance, if you find yourself creating the same kind of tasks over and over, you might want to consider purchasing a templating plugin. If you have complex deliveries, you might need some kind of portfolio tool.
The thing with JIRA is that as you start using it, you'll want to use integrations of other tools with it. You'll want your documentation linked to your issues, you'll want to track your code commits from JIRA, you'll want to review your code. Conveniently, Atlassian is always there to sell you a piece of software that does just that, is very reasonably priced at low user tiers and integrates well with JIRA and before you know it, you got a whole bunch of Atlassian tools. And then you onboard more users and now you have to upgrade your license and now it starts costing real money, but you're hooked. Confluence, Bitbucket and HipChat were instant hits at my org.
But overall, if I could suggest a few free / relatively inexpensive plugins I consider to be a must for whatever kind of organization you have:
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