JavaScript help needed..

I need to calculate TOTAL MILES wherein Start Mileage is subtracted from End Mileage (giving net mileage). I am not familiar with JavaScript and I've poked around their tutorials for days now with no luck. How do I calculate these fields when both Start and End "fillable form" numbers will change?
Do I use the Permissions label in the calculation? how do I identify that field ?.... All help is appreciated ..
