The first admin username and password is specified by you during the setup wizard (see https://confluence.atlassian.com/crowd/running-the-setup-wizard-192479.html#RunningtheSetupWizard-Step8.DefaultAdministrator)
If you know the username or email address you used for that user, you could recover the account by using the "Forgot your password?" link. Otherwise, you'll either have to uninstall/reinstall or follow Atlassian's password recovery doc: https://confluence.atlassian.com/crowdkb/restoring-passwords-to-recover-admin-user-rights-283642653.html
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