Making tool in arcmap by using modelbuilder or python

I am trying to making one tool by using python. For that I already make GUI. In that i have 5 inputs. These 5 inputs are feature layers(shape files). Now I make another option field. It is multiple choice option.

Now I want that all 5 input feature layer's fields that are add into this field option. From which I select some fields for further process.

I am using arcmap9.3, I make one toolbox and add my script as tool in that.

How can i do this?