Default selection for within

<h:selectOneMenu id="items" value="#{bean.selectedItem}">
  <f:selectItem itemLabel="10" itemValue="10"/>
  <f:selectItem itemLabel="20" itemValue="20"/>
  <f:selectItem itemLabel="30" itemValue="30"/>
</h:selectOneMenu>

The default selection would be the one which has value same as selectedItem which you set in bean.

selectedItem = 20;