The Authentication docs explain the process https://dev.twitch.tv/docs/authentication
Which auth flow you want to use depends on your use case. If you’re making requests from your server and don’t need any scopes then you can use the Client Credentials flow to get an App token. If you need user scopes for the endpoints you wish to access you’ll either need to use the Implicit flow if you’re making requests client-side, or the Auth Code flow if you’re making requests server-side.