Set a dataCombo selection

Hi 

How can I set a default value in a dataCombo?

I  tried 
ClientMethod onloadHandler() [ Language = javascript ]
{

 zenPage.getComponentById('mycombo').setProperty('selectedIndex',0);

 //zenSetProp('mycombo','selectedIndex',0);

}

and didn't work, I can not use the value property because I don't know what the values in my combo are going to because they loaded  OnCreateResultSet,  I'm only looking to make always the first value selected by default 

Thanks