You'd want to use CSS 3.
http://border-radius.com/ Can give you the code or you can use this:
ALL CORNERS:
-moz-border-radius: 10px;
-webkit-border-radius: 10px;
border-radius: 10px; /* future proofing */
-khtml-border-radius: 10px; /* for old Konqueror browsers */
INDIVIDUAL CORNERS:
-moz-border-radius-topleft: 10px;
-moz-border-radius-topright: 20px;
-moz-border-radius-bottomright: 30px;
-moz-border-radius-bottomleft: 0;
-webkit-border-top-left-radius: 10px;
-webkit-border-top-right-radius: 20px;
-webkit-border-bottom-right-radius: 30px;
-webkit-border-bottom-left-radius: 0;
SHORTHAND: -moz-border-radius: [top-left] [top-right] [bottom-right] [bottom-left] SO....
-moz-border-radius: 10px 20px 30px 0;
ELLIPTICAL ROUNDING (FIREFOX 3.5+): -moz-border-radius-topleft: [horizontal radius] [vertical radius]; SO....
-moz-border-radius-topleft: 10px 40px;
Got these codes from here:
http://css-tricks.com/snippets/css/rounded-corners/