HOWTO: Dynamically Create A Form

I know this sounds very simple, but I only want certain fields on my form to be displayed depending on the selected item in a drop-down menu, that is part of the form. I cannot figure out how to get it to work, and I don't want to resubmit the form... *** I need it to be done on the fly.*** Your help is appreciated! Example of online form

        Source of Income: _________________
        Dues:             _________________
        Type of Funds:    ________________v

        Where ____ = text field
                ___v = drop-down menu

Based on the selection of "Type of Funds" I want one more field to be displayed. pseudo-code:

        if (type_if_fund == 'Bank') {
                // then display text field to
                // enter the bank's name
        } else {
                // no need to display anything else
        }

Thanks again. -Scott