Calculating fields on a form (1 Viewer)

I have a MySQL database on my site that I use to keep track of my car's gas mileage. I imported the data from the Excel spreadsheet I was using, so the majority of the entries have the MPG already calculated. My problem is that I cannot figure out how to do calculations on data that I'm inputting through a form.

I've never messed with MySQL or with any in depth PHP, so please bear with me.

You can see the data here: http://www.cheesepatrol.com/gasmileage/

Oh, is there any way to lock the headers so that they scroll down with the document?

Thanks