How to customize magento onepage checkout form

Navigate to your theme folder. The default file is at location:

app\design\frontend\base\default\template\checkout\onepage\shipping.phtml

You might need to change multiple files:

As above:

app\design\frontend\base\default\template\checkout\onepage\shipping.phtml

but also...

app\design\frontend\base\default\template\checkout\onepage\billing.phtml

and also...

app\design\frontend\base\default\template\persistent\checkout\onepage\billing.phtml

Tada!