How to read and convert the value from listbox GUI?

I am trying to write a simple app converting units for example from kilograms into grams. The user is choosing from a listbox the unit he enters and writing the value in box below. The user is also choosing what unit the value will be converted into from the listbox and the effect of convertin is showing in the another box. How to write a commend which will read which units chose the user form the listbox and how to convert it? Or maybe you know some helpful applications ?

[Walter Roberson]

GUIDE, or App Designer ?

[Alex Boj]

App Designer