<meta http-equiv="Content-Type" content="text/html; charset=gb2312" />
<script language="javascript">
//--->
function checksubmit()
{
  if (document.form1.myname.value=="")
        {
           alert("请输入开始数值！");
           document.form1.myname.focus();
           return false;
        } 
        return true;  
}
//--->
</script>

<form name="form1" method="post" action="#" onsubmit="return checksubmit()">
  <label>
  <input name="myname" type="text" id="id">
  </label>
  <p>
    <label>
    <input type="submit" name="Submit" value="提交">
    </label>
  </p>
</form>