RESTful API best practices

I'm an Android Developer and also have some knowledge of RESTful API development. Currently I'm building an API using Dot Net Web API. Everything's working fine but I'm still concerned with the design. The core concern for me now is what exactly should be the Metadata of the response like what other helpful elements I can add apart from 'Response Code' and 'Response Message'. Second concern for me is that should the request be asynchronous or the query or both. And the last concern is what technique should I use to make a stateful communication like JWT or Basic Authentication, etc.