Insert Sum of Two fields In The Same Record Into Third Field

I have a table of customer orders with fields something like this:
customer : order-number : item1 : item1cost : item2 : item2cost : total-cost
I want the total-cost field to be the sum of item1cost and item2cost.
Do I use a query or can I do it in a formula?
Can I do it in my order entry form?
Can I create a dynamic total-cost field that is the sum of the item1cost and item2cost fields and updates when those fields are changed?
Please try not to smirk.
I'm a beginner.
I downloaded this week.
customer : order-number : item1 : item1cost : item2 : item2cost : total-cost
I want the total-cost field to be the sum of item1cost and item2cost.
Do I use a query or can I do it in a formula?
Can I do it in my order entry form?
Can I create a dynamic total-cost field that is the sum of the item1cost and item2cost fields and updates when those fields are changed?
Please try not to smirk.
I'm a beginner.
I downloaded this week.