How do I auto-fill data in Microsoft Access 2003?

Question

I am creating a Purchase Order Form at work using Microsoft Access 2003 so that it can be saved and archived in a database and reduce paper costs.

These are my current dilemmas:

1. I am unable to figure out how to auto-fill certain fields in the form. For example, I have an PO# Field which will serve as the ID section of the form. I want this to be auto-filled with each new form so that the first form created will have 001 and the second form created will have 002. I haven't been able to figure this out.

2. I have a section that has Quantity, Unit, Description, Unit Price, Price Total etc. How do I get Price Total automatically filled by doing the following: Quantity * Unit Price

3. I have Subtotal, S&H, Sales Tax, and Total. How do I get Total to be the sum of all these things?

I have been trying to figure these out using queries and tables, but I can't get it to work. I am currently using Microsoft Access 2003 in Form Design View.