how to set a username not availbale in asp.net and c#?

Post your user name to server when your text field changes focus and in server side query the database for checking the existence of user name if user name don't exist just send "OK" in json format else send any thing except "OK"

JQuery code is provided and i think you are able to write server side code

$('#YourUserNameField').blur(function () {
    var value = $('#YourUserNameField').val();
    $.ajax({
           url: '/Web_Method_To_Check_UserName/?uName='+value,
           type: 'post',
           success: function (data) {
           if (data.Result == "OK") {
               _canSubmit = true;
               $('#your_user_name_error_label').hide();
           }
           else {
               _canSubmit = false;
               $('#your_user_name_error_label').show();
           }
        },
        error: function () {
           alert("Error Occured");
        }
    });
});