form.user.focus()

   <script language="javascript">
      function SendForm () 
      {
          if(CheckPost()) 
        {
            document.addForm.submit();
       }
  }

function CheckPost () 
{
     if (addForm.user.value == "") 
     {
           addForm.user.focus();
           return false;
     }
     if (addForm.title.value.length < 5) 
     {
           addForm.title.focus();
           return false;
     }
     return true;
}
  </script>

  <form action="test.php" method="post" name="addForm">
       <div>用户:<input type="text" size="10" name="user"  maxlength="20"/></div>
       <div>标题:<input type="text" name="title" maxlength="50"/></div>
       <div>内容:<textarea name="content" rows="8" cols="30"></textarea></div>
       <div><input type="button" name="submit" value="发表留言" onclick="SendForm();"/></div>

</form>