This code is for dependend pick list using passing parameter in api. I need to do is using Jquery how to implement and by default value will be all category list in picklist.
When i reload this page by default >All Partners >All Categories >Title video will display 

Visualforce Page:-
 

<apex:page controller="categories" readOnly="true">


    <apex:form id="frm">

     <script>
    function prepareList(parent,child,list,isSubselectOptional){
        $("body").append("<select style='display:none' id='"+parent+child+list+"'></select>");
        $('#'+parent+child+list).html($("#"+list+" option"));
        $('#'+list).html("<option> — </option>");
        $('#'+parent).change(function(){
            var parentValue = $('#'+parent).attr('value');
            $('#'+list).html($("#"+parent+child+list+" .list"+parentValue).clone());
            if(isSubselectOptional) $('#'+list).prepend("<option> — Select — </option>");
        });
    }
    $(function() {        
        prepareList('PartnerId','ContentCategoryId',list, false);
    });
  

</script>


    <script>
   function myJavaMethod(){
      myactionfun();
  }
  </script>
 
 

     
       
     
    
    <apex:pageBlock >
             <apex:pageblockSection columns="3" >
                  <apex:pageBlock >
    
 
              <apex:outputPanel id="testPanel">
                 <Apex:selectlist size="1" value="{!PartnerId}" id="PartnerId" > 
                
                <apex:selectOptions value="{!company}"/> 
                   <apex:actionSupport event="onchange" >
                    </apex:actionSupport> 
                </apex:selectlist>
                   </apex:outputPanel> 
         
                </apex:pageBlock>
        
         
        
    
    
    
  <apex:outputPanel id="testPanel1">
         <apex:pageBlock >
             

                 <apex:selectList size="1" value="{!ContentCategoryId}" id="ContentCategoryId" onclick="myJavaMethod()" >   
                <apex:selectOptions value="{!items}" /> 
                <apex:actionSupport event="onchange" reRender="testPanel">
                </apex:actionSupport>  
                 
                </apex:selectList>
           
               </apex:pageBlock> 
               
         </apex:outputPanel> 
          <apex:outputPanel id="testPanel">
         <apex:pageblock id="pgBlock2"> 
          <!--  <c:PageBlockTableEnhancerADV targetPbTableIds="a"  pageSizeOptions="5,10,15,30,40,50,60,70" defaultPageSize="50" enableExport="false"/> -->
            <apex:pageBlockTable id="list" value="{!performcallout}" var="wrap" width="100%"  rendered="{!normalList}">
            <apex:column headerValue="Title" value="{!wrap.Title}"/>
            <apex:column >
            <apex:inputCheckbox value="{!wrap.selected}" > 
            </apex:inputCheckbox>
            </apex:column>
            </apex:pageBlockTable> 
        </apex:pageBlock> 
        </apex:outputPanel>   
         </apex:pageBlockSection>
         </apex:pageBlock>
 
    </apex:form>
    

</apex:page>

Apex Page :--
 

Public  class categories{
   
    public String ContentCategoryId {get;set;}
    public String PartnerID {get;set;}
  //  public String parentcompany_id{get;set;}
   // PUBLIC List<selectOption> Items;
    public String toAddresses {get; set;}
    
    public   String MySessionID = UserInfo.getSessionID();
    public   String myurl= URL.getSalesforceBaseUrl().toExternalForm() + '/services/Soap/c/10.0/' + UserInfo.getOrganizationId();    
    public   string c='eU9WzoFgU4n8Apu5PYxcNGRZswRDZJWDEMdbQVU85gw=';
    public   String Acode= Userinfo.getuserid();
    public   String Oid = UserInfo.getOrganizationId();  
    //phil  
    // Phill public   String Acode= '00530000000rLMxAAM';
   // Phill public   String Oid = '00D300000006YSZEA2';
   //bit9
   // public   String Acode= '00534000008cnnUAAQ';
   //  public   String Oid = '00D28000001G0V0EAK';
    Blob cryptoKey= EncodingUtil.base64Decode(c);
    
    Blob data = Blob.valueOf(MySessionID );
    Blob data1 = Blob.valueOf(myurl);
    Blob data4 = Blob.valueOf(Acode);
    Blob data5 = Blob.valueOf(Oid);
    
    Blob encryptedData = Crypto.encryptWithManagedIV('AES256', cryptoKey, data);
    Blob encryptedData1 = Crypto.encryptWithManagedIV('AES256', cryptoKey, data1);
    Blob encryptedData4 = Crypto.encryptWithManagedIV('AES256', cryptoKey, data4 );
    Blob encryptedData5 = Crypto.encryptWithManagedIV('AES256', cryptoKey, data5 );
    
    public    String b64Data = EncodingUtil.base64Encode(encryptedData);
    public    String b64Data1 = EncodingUtil.base64Encode(encryptedData1);
    public    String b64Data4 = EncodingUtil.base64Encode(encryptedData4);
    public    String b64Data5 = EncodingUtil.base64Encode(encryptedData5);
    
    public String aucode= EncodingUtil.urlEncode(b64Data4, 'UTF-8');
    public String ocode= EncodingUtil.urlEncode(b64Data5, 'UTF-8');
    
    public String ssnid{get;set;}
    public boolean normalList{get;set;}
    public boolean selectedList{get;set;}
    public String Title{get;set;} 
    public Boolean selected {get; set;} 
    
    public List<videolist> ConsoleWrapperList5{get;set;}
    public string productName{get;set;}
    public integer partner_count{get;set;}
    
    public categories() {
    
   
  // this.ContentCategoryId = 'defaultValue';

       
        getperformcallout6();
        getperformcallout5();
         getperformcallout();
        normalList = true;
        selectedList = false;
     
    }
    public List<companylistwrap> ConsoleWrapperList6{get;set;}
   
    public List<companylistwrap> getperformcallout6(){ 
        ConsoleWrapperList6 = new List<companylistwrap>(); 
        HttpRequest req = new HttpRequest();
        HttpResponse res = new HttpResponse();
        Http http = new Http();
        System.debug('Calling method now' );
        string url = 'https://www.demomail.net/sf/api/PartnerList?AUCode='+aucode+'&SOrgID='+ocode+'&CategoryId='+ContentCategoryId+'&LanguageId='+0+'&PartnerId='+PartnerID;
        //string url = 'https://www.demomail.net/sf/api/PartnerList?AUCode=GlrI5Pb2xh0v3GzK%2FeE8jOX2IG8Kt9lEuzKMJAkgSeiQ6xvUbcgCFlGVG3YURHvJ&SOrgID=Srx%2F%2BRAw%2B9QTiQvLe1h4GBTKvDo3lUBrBU9ksiTTvlW8OWdlQE3Hsj1QQwkgqypL&PartnerId=PartnerId;
        req.setEndpoint(url);
        System.debug('&&: '+ url );
        req.setMethod('GET');
        res = http.send(req);
        if(res.getstatusCode() == 200 && res.getbody() != null)
        {
            ConsoleWrapperList6=(List<companylistwrap>)json.deserialize(res.getbody(),List<companylistwrap>.class);
        }
        return consolewrapperlist6; 
    }
    public List<SelectOption> getcompany() {
        List<SelectOption> options = new List<SelectOption>();
                   
                  // options.add(new SelectOption('null','-Select-'));
                    options.add(new SelectOption('0','All Partners'));
                    
        for(companylistwrap crRec : ConsoleWrapperList6){
        
        options.add(new SelectOption(crRec.PartnerID, crRec.PartnerName));
               
  
        } 
        
        
          partner_count= options.size();
          getperformcallout5();
          System.debug('##: '+  partner_count);
        return options;
    }
    public List<videolist> getperformcallout5(){ 
        ConsoleWrapperList5 = new List<videolist>(); 
        HttpRequest req = new HttpRequest();
        HttpResponse res = new HttpResponse();
        Http http = new Http();
        System.debug('Calling method now' );
   
      // string url = 'https://www.demomail.net/sf/api/ContactCategoryList?AUCode=GlrI5Pb2xh0v3GzK%2FeE8jOX2IG8Kt9lEuzKMJAkgSeiQ6xvUbcgCFlGVG3YURHvJ&SOrgID=Srx%2F%2BRAw%2B9QTiQvLe1h4GBTKvDo3lUBrBU9ksiTTvlW8OWdlQE3Hsj1QQwkgqypL&CategoryId=4788&LanguageId=0&PartnerId=1227';
        string url = 'https://www.demomail.net/sf/api/ContactCategoryList?AUCode='+aucode+'&SOrgID='+ocode+'&CategoryId='+ContentCategoryId+'&LanguageId='+0+'&PartnerId='+PartnerID;
        req.setEndpoint(url);
        System.debug('**: '+ url );
        req.setMethod('GET');
        res = http.send(req);
        if(res.getstatusCode() == 200 && res.getbody() != null)
        {
            ConsoleWrapperList5=(List<videolist>)json.deserialize(res.getbody(),List<videolist>.class);
        }
        return consolewrapperlist5; 
    }
    public List<SelectOption> getItems() {
        List<SelectOption> options = new List<SelectOption>();
                  
                    
            for(videolist crRec : ConsoleWrapperList5){
                options.add(new SelectOption(crRec.ContentCategoryId, crRec.CategoryName));
                System.debug('##: '+ crRec.ContentCategoryId);
            }  
            partner_count= options.size(); 
             getperformcallout();
        return options;
       
    }
    public List<consolewrap> ConsoleWrapperList{get;set;}
    public  List<consolewrap> getperformcallout(){
        ConsoleWrapperList = new List<consolewrap>();
        HttpRequest req = new HttpRequest();
        HttpResponse res = new HttpResponse();
        Http http = new Http();
        string url = 'https://www.demomail.net/sf/api/SendVideoContact?AUCode='+aucode+'&SOrgID='+ocode+'&CategoryId='+ContentCategoryId+'&LanguageId='+0+'&PartnerId='+PartnerID;
        System.debug('@@: '+ url);
        req.setEndpoint(url);
        //System.debug('!!!'+ SessionId);
        req.setMethod('GET');
        //system.debug('???'+aucode);
        //system.debug('***'+ocode);
        res = http.send(req);
        // System.assert(false,res.getBody()+'-----');
        if(res.getstatusCode() == 200 && res.getbody() != null){
            ConsoleWrapperList=(List<consolewrap>)json.deserialize(res.getbody(),List<consolewrap>.class); 
            System.debug('value: '+ ConsoleWrapperList );
        }
        
        return ConsoleWrapperList;
    } 
    
    public pageReference getperformcalloutpageReference ()
    {
        getperformcallout(); 
        return null; 
    } 
    
    public pageReference getcompanypageReference ()
    {
        getperformcallout5();
        return null; 
    }  
    
     
}