Creating Proceedure

Very basic query here.
I am creating a Purchase order form using Access forms and tables etc. Included in this form is a combo for TAX rate,Unit Price, TAX cost and Total price.

The user will select the Tax rate and put in unit price. Once both are selected/entered I want to calulate the TAX per unit automatically and put in in DB.

Whats the best way to automate this? Is it using a command button. How do I code this Event through use of button?

use the control wizard