@aronaman theres not much on Laracasts on api creation but this one (whilst being old) is still a good watch https://laracasts.com/series/incremental-api-development
As regards to postman, once you’ve downloaded and register it, you will be prompted to some resources and tutorials about how to define refine requests and write api tests.
As for the specifics of your ERP, you’ll be best asking the owner what endpoints they are expecting to get data from and also mocks of the responses they are expecting and then you can build out your endpoints according to this info,