Check out border-radius
But not all browser support it so you need javascript also.
Screenr - @NETTUTS: How to make rounded corners with CSS and JavaScript.

You could just say something like this in your stylesheet (I've laid out all corners separately so you can manipulate them separately).

However, as has been pointed out already, this is not necessarily supported by older browsers, so here are some less obvious ways of doing it that would be more widely supported.

Hope this helps!

The border-radius style is not fully supported by all browsers. For now, you should include all of these styles:

border-radius
-moz-border-radius
-webkit-border-radius

These will work in most of the popular modern browsers, but NOT in IE 8 or less.