Set default option on configurable products in magento

Open app\design\frontend\[your package]\[your theme]\template\catalog\product\view\type\options\configurable.phtml

Now add the below java-script code after :-
var spConfig = new Product.Config(<?php echo $this->getJsonConfig() ?>);


function fireEvent(element,event){
if (document.createEventObject){
// dispatch for IE
var evt = document.createEventObject();
return element.fireEvent('on'+event,evt)
}
else{
// dispatch for firefox + others
var evt = document.createEvent("HTMLEvents");
evt.initEvent(event, true, true ); // event type,bubbling,cancelable
return !element.dispatchEvent(evt);
}
}
Event.observe(window, 'load', function() {
spConfig.settings[0].selectedIndex = 1;
obj = spConfig.settings[0]; // this grabs the first select item
Event.observe(obj,'change',function(){});
fireEvent(obj,'change'); // this simulates selecting the first option, which triggers
spConfig.settings[1].selectedIndex = 1; // this selects the first option of the second attribute drop menu
});