How do I use/create a simple modal popup?

I'm looking for a simple way to create / use a modal popup on my page either with a <asp:Button> or a an <asp:LinkButton>. I just need some simple examples and or code to do this. On the popup will probably be a text box and a button to do a quick check against a value in a table in my db and then upon confirmation / validation of this value, close the popup.