Need a little scripting help....

I am developing a smiple PDF quote form and need help in scripting a basic calculation.... basically taking a product of two fields and subtracrting any discount if given.... basically the formula is QTY.0 x RATE.0 - DISCOUNT.0 = VALUE. Every field will be a variable. I am not a programmer and have no idea how to create this custom script... some help would be appreicated. - Thanks!