How to access the serialized data from inside web mothod using Ajax jquery form post using web methods c# asp.net

Html

        <div class="form-group">
    <label for="firstName">First Name:</label>
    <input type="text" class="form-control" id="firstName" name="firstName"  runat="server"/>
  </div>
  <div class="form-group">
    <label for="lastName">Last Name:</label>
    <input type="text" class="form-control" id="lastName" name="lastName"  runat="server"/>
  </div>
  <div class="form-group">

<input type="file" Class="form-control" id="fileUpload" name="fileUpload" runat="server" />


  </div>
  <input type="button"   id="submit"  class="btn btn-default" runat="server"  value="Submit"/>




    </form>

Javascript

        $(document).ready(function(){
        $("#submit").on("click", function (e) {


            //   alert($("form").serialize());
            var obj = '{"firstName":' +'"'+$("#firstName").val() + '"'+', "lastName":' + '"'+ $('#lastName').val() + '"}';
            console.log(obj);
            $.ajax({
                type: "POST", url: "WebForm1.aspx/CreateUser", data: obj /*$("form").serialize()*/
               ,
            contentType: "application/json; charset=utf-8",
            dataType: "json",
                async: true,
                success: function (data, status) {
                    console.log("Call Success");

                    alert(data.d);
                },
                failure: function (data) {
                    console.log("Call Fail");
                    alert(data.d);
                },
                error: function (data) {
                    console.log("Call Error");
                    alert(data.d);
                }
            });

        });

        });

Web Method

[System.Web.Services.WebMethod]
        public static int CreateUser(String firstName, String lastName)
        {
            Users userObj = new Users();

            userObj.fileName = "xyz";// user.fileName;//fileUpload.PostedFile.FileName;
            userObj.firstName = firstName;//firstName.Value;
            userObj.lastName = lastName;//lastName.Value;
           // fileUpload.SaveAs(Server.MapPath("~/") + "/Uploads/" + fileUpload.PostedFile.FileName);
        //    System.Diagnostics.Debug.WriteLine( Server.MapPath("~/") + "/Uploads/" + fileUpload.PostedFile.FileName);
            SQLDAL sqldalobj = new SQLDAL();

            int success = sqldalobj.Create(userObj);

          //  System.Diagnostics.Debug.Write(user);
            return success;

        }

Right now I am able to pass text box values as json data and inside web method its received correctly.But when I need a larger number of text boxes using current approach its would result in large number of parameters which I want to avoid using a object as parameter instead of different parameters for each html form element.And using this approach I am failing upload the file .So my question is if I serialize the form and send the data to webmethod how to properly recieve the said serialized form and retrieve the data from it including the fileupload