auto running a macro when a cell value changes

This is driving me nuts! It is mainly due to the fact that I am a new user to this. I have a workbook that contains several worksheets. I have the correct formulas in place, and I have created a Macro (Called Test_1) that seems to work so far. How ever I would like the Macro to run when cells G12 through G122, and H12 through H122 (these are on a work sheet named Inv.) have a value greater than 0. Or are changed from their current value. I have tried several times to get the VB code correct, but I just keep missing something. Any suggestions?

Thank you so much this is a giant leep into the right direction.

This code "kinda" works. It actually causes a few more questions.
1.Can you run a macro in the background so that you can't see it?

2.Can you run this VB code on a column that has a formula? So that the variable can change without causing the macro to run, but when cells I12 through I122 change (these are the sum of G's and H's) it will cause the macro to run?

3.When this code executes the macro it displays the last worksheet in the workbook. This can mess you up especially considering that you change the values, and therefore are looking at, the first worksheet in the workbook when the macro runs. Is there a way to stay on the first worksheet?