What APIs do I need?

[Fromm60056]

I am new to Shopify (but an long time AS/400 programmer.

I am trying to interface our ERP system with Shopify. I can use Postman to create Rest APIs to retrieve orders, but once I retrieve the orders, how do I change their status so I don't pick them up again?

Question 2 is shipping. Is this called Fulfillment? What APIs do I need to tell Shopify that I have shipped the order in my ERP, this includes item quantities and UPS tracking #s. 

So big picture, what APIs are required to ship an order?

Do I have to create an APP? even though I am just using APIs?