HTML onmousedown/onclick display hidden HTML

I'm making a form to order different varieties of sweets from a website. At the moment I have checkboxes with each variety. Once they have chosen the varieties they want (they can have more than one) then I need some more information. To do this I want to display a new box when they check each checkbox. Event attributes seem to be adequate, but I don't really know javascript, so is this the right way for me to do it? Can event attributes only trigger javascript?

Or perhaps I'm going about this the wrong way, would there be a better way to make this form? I've considered a shopping cart but for what I want I think it's too much, and I'm not very advanced.

So, I just want a way to show html after a checkbox has been ticked, or a better way to make my form.

Thanks