Welcome to the RingCentral Community

Once you singned into developer portal here https://developer.ringcentral.com/

You will be given a sandbox environment as default to create different applications and test APIs there. You can go through the link given above to start with.

Once the application you created pass all the criteria and rules in the sandbox, you can push that to production later

Once you have authenticated into the Developer Portal (https://developer.ringcentral.com) you can click on your name/avatar and then click on "Sandbox Accounts" to create your Sandbox Account to begin development.

Once you have done that, continue with the tutorial URL which Brandon shared above.

I would start here: https://developer.ringcentral.com/ and here: https://developer.ringcentral.com/library/tutorials/test-account.html in particular