You would have to create a module by clicking on module on the left of the window. This allows you to write vba (Visual Basic for Applications) code. Create a function called TaxCalc or anything else you want to name it and make it public. That way it can be reached by queries, etc.
Public Function TaxCalc(Income as single) as single
your code here
This accepts the income as a parameter and calculates a result. If you name the result TaxValue, your last statement in the function would be