To use GraphQL in WordPress you have to install the wp-graphql plugin.
To install this plugin You have to download it from GitHub: download wp-graphql
If you don’t know how to install a WordPress plugin manually follow this tutorial
After Activating this plugin you can visit https://yoursitename.com/graphql to access the GraphQL end-points. Here you can do all the queries and mutation.
If you want to access the GraphQL Playground you have to install this plugin: download WP-GraphiQL
If you want to read things in very detail: read here