[SalesForce] How to retrieve & insert the success “id” from web service response

I have got a web service provided to me, which I use & insert a record into the external application through Salesforce "Task" object. As a response, the following is returned by the web service
{"status": "ok","message": "Successfully added new note.","data": [],"id": "12345"}.

Now, I got the requirement to capture the response "id" and insert into Salesforce "Task" custom field "noteId". And My Main web service class is

public with sharing class InsertNote_WebService 
{   
@future(callout=true)
public static void createNoteRecord(String memberId,String enteredBy,String description,string notecategory)
{
    Http h = new Http();
    HttpRequest req = new HttpRequest(); 
    HttpResponse res = new HttpResponse();
    String BaseUrl = 'http://testsite.com/service/id/key'; //My external website url
    JSONGenerator_AC jsgenrator = new JSONGenerator_AC();
    string Message = jsgenrator.generateJSONContent(memberId,enteredBy,description,notecategory);                                 
    req.setEndpoint(BaseUrl);
    req.setMethod('POST');
    req.setHeader('Content-Type','text/html');        
    req.setBody(Message);
    try{
        res = h.send(req);
    }
    catch(Exception e){
        System.debug(e);
    }
} 

My JSONGenerator_AC class is:

public with sharing class JSONGenerator_AC 
{
    public class Noteclass 
    { 
        String member_id;
        String entered_by;
        String note_text;
        String category_id;

        public Noteclass(String memberid, String entered_by, String description, string notecategory) 
        { 
            this.member_id = memberid;
            this.entered_by = entered_by;
            this.note_text = description;
            this.category_id = notecategory;
        }
    }
    Noteclass note;
    public String generateJSONContent(String memberid, String entered_by, String description, string notecategory) 
    {
        // Create a JSONGenerator object.
        // Pass true to the constructor for pretty print formatting.       
        JSONGenerator jsonGen = JSON.createGenerator(true);

        // Create an object to write to the JSON string.        
        note = new Noteclass (memberid, entered_by, description, notecategory);

        // Write data to the JSON string.
        jsonGen.writeStartObject();
        jsonGen.writeStringField('member_id', memberid);
        jsonGen.writeStringField('entered_by', entered_by);
        jsonGen.writeStringField('note_text', description);
        jsonGen.writeStringField('category_id', notecategory);
        jsonGen.writeEndObject(); 

        // Get the JSON string.
        String jsonstring = jsonGen.getAsString();
        System.debug('Hello Sample JSON'+ jsonstring);
        return jsonstring;
    }
    /*
    *@decsription: Utility Method to generate response JSON
    */
    public static String generateResponseJSON(Map<String,String> responseValues)
    {
        JSONGenerator jsonGen = JSON.createGenerator(true);
        jsonGen.writeStartObject();
        jsonGen.writeFieldName('response');
        jsonGen.writeStartObject();
        for(String key:responseValues.keySet())
        {
            jsonGen.writeStringField(key,responseValues.get(key));
        }
        jsonGen.writeEndObject();
        jsonGen.writeEndObject();
        return jsonGen.getAsString();
    }

    /*
    *@decsription: Utility Method to generate fault JSON
    */
    public static String generateFaultJSON(String faultCode, String faultMsg)
    {
        JSONGenerator jsonGen = JSON.createGenerator(true);
        jsonGen.writeStartObject();
        jsonGen.writeFieldName('response');
        jsonGen.writeStartObject();
        jsonGen.writeStringField('errorCode',faultCode);
        jsonGen.writeStringField('message',faultMsg);
        jsonGen.writeEndObject();
        jsonGen.writeEndObject();
        return jsonGen.getAsString();
    }
}