Javascript Remoting through HTML in salesforce

https://developer.salesforce.com/page/Apex_Web_Services_and_Callouts Check this out. This is a very useful blog.

I believe you have an external page and you are trying to access an apex method. For this the apex class must be a web service class. This means a global class exposed. like this

 global class AccountPlan {

   webservice static Plan createAccountPlan(Plan vPlan) {

           //add logic here......
   }
 }

And from your HTML page you can call this method using AJAX calls.

Hope this was useful

Try something like this :

VF :

<div id="header">
    <script language="javascript">
        var myControllerMethod = '{!myControllerMethod}';
    </script>
</div>

Controller :

 public with sharing class MyClass {
        public String getMyControllerMethod () {
            //Do some stuff

            return null;
        }
    }