Where should I start from?

Before creating the search endpoint one needs to setup server, index and processors.

For most of the basic cases this index config would be enough:

Screenshot of search API index configuration page

Of course any other fields could be added, that are project specific.

If database backend is used it is important to use processors (/admin/config/search/search-api/index/<you_index_machine_name>/processors), because this backend is not smart enough to do all of those processing on its own in contrary to Solr, Elastic and others. This is an example of what processors could be enabled in your project:

Search API Index Processors