Maintaining Unique CustomerID With Multiple Purchases

I am trying to make a basic database system to track orders and contacts and the like for my small business. I have created a series of databases that look pretty standard, something like this example:

I want to create a form that I will open and complete every time I process an order. I want to fill out both the customer info and the order details at one time.

Assuming that the "CustomerID" in the "Customers" table is set to Autonumber in order to create a unique ID, how do I keep from creating multiple new "customers" every time I enter a new order regardless of whether or not that customer already has an ID number from a prior order?

It seems that I would need to make some sort of Macro that would say something like, if the "CompanyName" entered is already listed in the 'Customer' table, use the existing CustomerID, if it is not listed in the Customer table create a new CustomerID.

Should I be looking into Macros, and if so should the macro be built into the form or the database?