[SalesForce] Change the label of “none” value of a picklist

There are 2 issues:

1) You have an extra 's' in the parameter that you're passing to the doChange function. Change the line to :

<apex:inputField value="{!opportunity.StageName}" onchange="doChange(this);"/>

2) You are toggling the visibility of the child element of the div(id="myId"), but the div itself remains hidden all the time. If the parent is hidden the child elements will always remain hidden regardless of their visibility setting.

So change the doChange function:

function doChange(thiss){
   if(thiss.value=='Closed Lost'){
      document.getElementById("myId").style.display='block';
   }else{
      document.getElementById("myId").style.display='None';
   }

}

Just go to the Picklist definition on the custom object and select Reorder and it allows you to set a Default value.

Build>Create>Objects>My Object>Custom Fields

Then on the page layout make the picklist required so it will not permit a value of the Salesforce default 'none'.

Build>Create>Objects>My Object>Page Layouts

This will mean that the standard Salesforce 'none' option on the picklist in the detail page is hidden.