You can use javascript for that.
<script type="text/javascript" language="javascript">
function validate
{
if (document.getElementById("<%=txtName.ClientID%>").value=="")
{
alert("Name Feild can not be blank");
document.getElementById("<%=txtName.ClientID%>").focus();
return false;
}
if(document.getElementById("<%=txtEmail.ClientID %>").value=="")
{
alert("Email id can not be blank");
document.getElementById("<%=txtEmail.ClientID %>").focus();
return false;
}
}
</script>
and use this function on your cs page like
btnSubmit.Attributes.Add("onclick", "return validate()")