in textbox write this code onkeypress="return isNumberKey(event)"
and function for this is just below.
<script type="text/javascript">
function isNumberKey(evt)
{
var charCode = (evt.which) ? evt.which : event.keyCode;
if (charCode != 46 && charCode > 31
&& (charCode < 48 || charCode > 57))
return false;
return true;
}
</script>