You can start using the GraphQL API immediately with no API key. In order to get higher rate limits and useful guides/examples you can sign up at:
API Endpoint: GraphQL Playground: API Docs: If you have an API key, it should be set as a x-api-key request header. i.e In the GraphQL playground, click HTTP HEADERS at the bottom and paste the following: { "x-api-key": "API-KEY-HERE" }