Inclusion in scholarly indexes is a powerful way to increase the discoverability of your journal. Types of indexes vary, from search engines that are freely available (e.g., Google Scholar), to those that index exclusively open access content (e.g., DOAJ), to subscription aggregator databases that are paid for by libraries and other institutions (e.g., Scopus), to subject specific indexes (e.g., PubMed Central). Each index will have its own set of requirements that a journal must meet before it can be listed.
While indexing might appear to be something to consider once the publication is up and running, it actually is ideal to identify desirable indexes as you are establishing your journal in order to learn what is required to be listed—for example, some indexes require a specific number of published issues before they will consider including a journal.
This blog post authored by Scholastica provides an overview and useful information on indexes and indexing. The PKP’s Getting Found, Staying Found, Increasing Impact guide also includes some helpful information on indexing.