Display popup message only once for new opp

Question

I have a requirement to display a popup message ONLY when the opp is created and it satisfies a condition (for instance, amount >100000). Im stuck here at the implementation of the javascript and how to invoke from apex:

Finally, got this to work:

VF page:

<apex:page standardController="opportunity"  extensions="oppMessage" >

<apex:form >
   <apex:actionFunction action="{!markread}" name="markread"/>
 </apex:form>


 <script>

 if ({!Opportunity.Amount}>10000 && {!!Opportunity.messageshown__c})   { 


             alert ("Contact Mr. X");

             markread ();

                                                                        }

 </script>


</apex:page>

Controller:

public  class oppMessage {

 ApexPages.StandardController controller;

 public oppMessage(ApexPages.StandardController controller) {
        this.controller = controller;
    }
    public void markread() {
        Opportunity o = (Opportunity)controller.getRecord();
        o.MessageShown__c = true;
        update o;
    }
}