There are many ways to do that.
Simplest in your case would be to check the length of the the text in the textbox. For length = 0 i.e first character, just don't allow 0 (key code 48) otherwise go with your normal validation. So change your code to:
function isSpaceKey(evt) {
var charCode = (evt.which) ? evt.which : event.keyCode
if (document.getElementById("TextBoxID").value.length == 0) {
if (charCode == 48) {
alert("Enter Numeral Values only!");
return false;
}
}
else {
if ((charCode < 47 || charCode > 57) && charCode != 8) {
alert("Enter Numeral Values only!");
return false;
}
}
return true;
}
Hope that helps!
Edit - I misread the question before. Updated the answer as per your requirement.