Suming web page form fields together

Hi, I'm pretty new to web design and need a bit of help please.

I have a form on a web page with a number of fields on it. The intention is that a user enter values in each field and then a total field is populated.

When I say a number of fields I mean 400!, so I'm open minded about whether the total is updated after each input or the total is calculated on the press of a button.

Please help, as I have no idea how to code this calculation!

Cheers, Neil.

All sorted now, found a 'teach yourself javascript in 7 days' pdf. Got through it in 10 hours at 3am this morning!!!