Magento Configurable Products - Setting Default Value for Options

Add the javascript hack at the bottom into template\catalog\product\view\type\options\configurable.phtml and automatically load the first option. copy below code under the var spConfig line:

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 );
return !element.dispatchEvent(evt);
}
}

function setDefaultConfigOptions()
{
if (spConfigIndex >= spConfig.settings.length)
{
return; // stop
}

spConfig.settings[spConfigIndex].selectedIndex = 1;
var obj = spConfig.settings[spConfigIndex];

++spConfigIndex;

Event.observe(obj,'change',function(){});
fireEvent(obj,'change');

window.setTimeout("setDefaultConfigOptions()", 1); // Add a small delay before moving onto the next option
}

var spConfigIndex = 0;
Event.observe(window, 'load', function() {
setDefaultConfigOptions();
}); 

Source: http://icebergcommerce.com/software/blog/iss/article/magento-configurable-products-setting-default-value-for-options