Thanks guys
I've done the validations using javascript and it worked my problem is I need to include the style that will change the background of the textbox if there's an error, here's my script:
<script language="javascript">
<!--
function isformvalid()
{
//Load Vars from form
txtFirst_Name = document.frmValidate.txtFirst_Name.value;
//alert(txtFirst_Name);
txtSurname = document.frmValidate.txtSurname.value;
txtEmail = document.frmValidate.txtEmail.value;
txtPostal_Code = document.frmValidate.txtPostal_Code.value;
msgHeader = "The form contains the following errors:\n\n";
errMsg = "";
numErrors = 0;
//First Name
if(txtFirst_Name==""

{
errMsg += "Your first name is required,\n";
document.frmValidate.txtFirst_Name.focus();
document.getElementById("firstNameError"

.className = "show";
numErrors = numErrors + 1;
} else {
document.getElementById("firstNameError"

.className = "hide";
}
//Surname
if(txtSurname==""

{
//alert("here"

;
errMsg += "Your Surname is required,\n";
//document.frmValidate.txtSurname.focus();
document.getElementById("surnameError"

.className = "show";
numErrors = numErrors + 1;
} else {
document.getElementById("surnameError"

.className = "hide";
}
//Email
if(txtSurname==""

{
//alert("here"

;
errMsg += "Your Email is required,\n";
//document.frmValidate.txtEmail.focus();
document.getElementById("EmailError"

.className = "show";
numErrors = numErrors + 1;
} else {
document.getElementById("EmailError"

.className = "hide";
}
//Postal Code
if(txtPostal_Code==""

{
errMsg += "Enter Postal Code is required,\n";
//document.frmValidate.txtPostal_Code.focus();
document.getElementById("CodeError"

.className = "show";
numErrors = numErrors + 1;
} else {
document.getElementById("CodeError"

.className = "hide";
}
//Check to see if we had any errors
if(numErrors>0){
alert(msgHeader + errMsg);
return false;
}else{
return true;
}
}
-->
</script>