need a trigger to enable text item dependant on list item

Hi ,

I have created a list item with some drop down items , apart from this , there are  two text items currently has property enabled to false (disabled to enter anything )

now my requirement is i when user select a lov item like "Pre-paid" ,  then the text item should be enabled to enter value ,

but i am not able to achive it , dont know which trigger to use , or if their is any other way to achive it .

Pls suggest .