You will need to set up a Confluence Cloud site for the migration.
Atlassian provide a free Cloud migration trial for server and data center customers considering a migration to Cloud. The trial lasts the duration of your server maintenance or data center subscription (for up to 12 months), and matches your current self-managed user tier (up to 20,000 users).
To migrate content you will need administrator permission in both the Confluence server instance and your Confluence cloud site.