Handler in asp.net processRequest

I created an Handler in my project, to call some functions.

Here is process request :

 public void ProcessRequest(HttpContext context)
    {

        context.Response.Cache.SetCacheability(HttpCacheability.NoCache);

        //Get basic parameters:
        string sAction = (string.IsNullOrEmpty(context.Request["action"]) ? "" : context.Request["action"]);


        switch (sAction) {
            case "GET":
                var res = class.GetClientList();
                break;
        }
    }

It works, but i'm beginner in web processes and i'd like to know, now i'm having my client list, what can i do with that? How can i get back the result of the request http://localhost:port/handler/api.ashx?action=GET ?