You need to create the content that will appear on the journal's About Page and to define review policies and procedures.
Your site will come pre-populated with form letters you can use in managing the review process. You can modify those letters to suit your needs.
Pro-tip: The help text on OJS pages can be dense and, at times, overwhelming. Persevere: the answer is usually there or in the extended documentation.
Open Access 101, from SPARC from Karen Rustad on Vimeo.