At first, the CRM application should be added to your Jira. Read how to find, add and manage the Server apps (Atlassian documentation).
The app doesn't work out of the box – it must be configured before, not to mention the projects, issues and workflows configuring required for the relevant process tuning. The CRM app settings are located at Jira Administration → CRM and are accessible for a user with Jira Admin rights.
The app provides some default settings – they described in the relevant sections of current documentation.
Speaking of rights in the context of CRM using, there are 2 main user types working with the application:
Jira Admin – configures the app, manages issues & workflows, defines the CRM access. Check the Admin's Guide to figure out how to set up the CRM and handle the particular use cases.
Others – regular Jira users who work with CRM entries directly in the CRM or within the issues. Check the User Guide to have an idea of CRM functionality for regular users.
You will also need to add a CRM superuser – the "technical" user with access to Jira to perform actions in the system.