Depends on your setup (determined by your administrators). Some systems are set up so that you can create your own accounts, others require you to create accounts in other systems (LDAP, Crowd, Atlassian ID, etc) and some require that you directly ask an admin to do it for you.
I'd check for the "sign up for an account" link on the page you land on when you visit the JIRA system you want access to, and if you can't see it, contact the admins.
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