list box pop-up and linking anyone?

I am creating a form and I'm trying to make so when a selection is made two things happen.
1. I want an alert box to pops up with instructions
2. I would like that selection to be a link to the text area box which comes next.

Example:

Yes /No list box --OnClick, Yes, alert box, ('please supply details') and also OnClick, yes link to text area box for detials

What I have tried has'nt worked. Eventhough I put the comand in the Yes tag it performs the alert action for both yes and no, and the link doesn't work at all.

any help?