How can I add a custom error message to my .aspx. Is there a way to make it template for other pages to use?

Here is how my .aspx page looks like:

<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="validation.aspx.cs" Inherits="Web.validation" %>

 <br>
< !DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<br>
< html xmlns="http://www.w3.org/1999/xhtml" > <br>
< head id="Head1" runat="server"><br>
    <title>Untitled Page</title><br>
< /head><br>
< body><br>
    < form id="form1" runat="server"><br>
    < div><br>
    < li><br>
            < label id="lblFirstName" for="FirstName"><br>
              First Name :
            < /label>
            < input id="FirstName" name="FirstName" type="text" maxlength="25" class="required" /><em><img
              src="images/required.png" alt="required" /></em> </li>
          <li><br>
            < label id="lbllastName" for="LastName"><br>
              Last Name :
            < /label><br>
            < input id="LastName" name="LastName" type="text" maxlength="25" class="required" /><em><img
              src="images/required.png" alt="required" /></em> </li><br>
          <li><br>
            < label id="lblAddr1" for="Addr1">
              Address :
            < /label><br>
            <input id="Addr1" name="Addr1" type="text" maxlength="25" />
          </li>
          <li>
            <label id="lblAddr2" for="Addr2">
              Address 2 :
            </label>
            <input id="Addr2" name="Addr2" type="text" maxlength="25" />
          </li>
          <li>
            <label id="lblZip" for="txtZip">
              Zip :
            </label>
            <input id="txtZip" name="txtZip" type="text" class="ZipCodeMask" />
          </li>
          <li>
            <label id="lblCity" for="City">
              City :
            </label>
            <input id="City" name="City" type="text" maxlength="25" />
          </li>
          <li>
            <label id="lblState" for="State">
              State :
            </label>
            <input id="txtState" name="txtState" type="text" maxlength="25" />
          </li>
          <li>
            <label id="lblPhone" for="txtPhone">
              Phone :
            </label>
            <input id="txtPhone" type="text" name="txtPhone" class="phone PhoneMask" />
          </li>
          <li>
            <label id="lblEmail" for="EMail">
              E-Mail :
            </label>
            <input id="EMail" name="EMail" type="text" maxlength="100" class="required email" /><em><img
              src="images/required.png" alt="required" /></em> </li>
          <li>
            <label id="lblComment" for="Comment">
              Comment or Question :
            </label>
            <textarea id="Comment" name="Comment" cols="40" rows="6" class="required"></textarea><em>
              <img src="images/required.png" alt="required" /></em> </li>
          <li>
            <ul>
              <li>
                <button id="btnCancel" name="btnCancel" type="button">
                  Cancel</button></li>
              <li>
                <button id="btnReset" name="btnReset" type="reset">
                  Reset</button></li>
              <li>
                <button id="btnSubmit" name="btnSubmit" type="submit">
                  Submit</button></li>
            </ul>
          </li>


    </div>
    </form>
  <script src="js/jquery.validate.min.js" type="text/javascript"></script>
</body>
</html>