<script language="javascript"
 type="text/javascript">
function validate()
{
if (document
.getElementById(
"<%=TextBox1.ClientID%>").value==""
)
{
alert("Name can not be blank"
);
document.getElementById
("<%=TextBox1.ClientID%>").focus();
return false;
}
if(document
	 .getElementById("<%=TextBox2.ClientID %>").
	 value=="")
{
 alert(
 "Email id can not be blank");
 document.getElementById
(
"<%=TextBox2.ClientID %>"
)
.focus();
return false;
 }
 var emailPat= /^([A-Za-z0-9_\-\.])+\@
	 ([A-Za-z0-9_\-\.])+\.([A-Za-z]
 {2,
 4
})$/;
 var emailid=
document.getElementById(
"<%=TextBox2.ClientID %>").value;
 var matchArray = emailid.match(emailPat);
 if 
 (matchArray == null)
 {
 alert();
 document.getElementById(
 "<%=TextBox2.ClientID %>").focus();
 return false;
 }
 if (document
	.getElementById
	("<%=TextBox3.ClientID%>")
	.value==
	"")
{
alert(
"Contact Number can not be blank");
document.getElementById(
"<%=TextBox3.ClientID%>").focus();
return false;
}
var digits=
	 "0123456789";
var temp;
for (
	 var i=0;i<
	 
	 document.getElementById(
"<%=TextBox3.ClientID %>").value.length;i++)
{
temp=document.
getElementById("<%=TextBox3.ClientID%>").
value.substring(i,i+1);
if (digits.indexOf(temp)==-
	 1)
{
alert("Please enter correct Contact Number"
	 );
document.getElementById(
	 "<%=TextBox3.ClientID%>").focus();
return false;
}
}
if(document
	 .getElementById(
	 "<%=TextBox5.ClientID %>").value==
	 "")
 {
alert("City can not be blank");
document.getElementById(
	 "<%=TextBox5.ClientID %>").value;
document.getElementById(
	 "<%=TextBox5.ClientID %>").focus();
return false;
 }
if(
	 document.getElementById(
	 "<%=TextBox6.ClientID %>").value==
	 "")
 {
alert("Password can not be blank");
document.getElementById(
	 "<%=TextBox6.ClientID %>").value;
document.getElementById(
	 "<%=TextBox6.ClientID %>").focus();
return false;
 }
 if(document
	.getElementById("<%=TextBox7.ClientID %>")
	.value=="")
 {
alert("Please Re-Enter Password ");
 document.getElementById(
 "<%=TextBox7.ClientID %>").value;
 document.getElementById(
 "<%=TextBox7.ClientID %>").focus();
 return false;
 }
return true;
}
</script>

Default.aspx Code

<%@ Page Language="C#"
 AutoEventWireup="true" 
Codevirtual="Default.aspx.cs"
 Inherits="_Default" 
 %>
<!DOCTYPE html PUBLIC 
"-//W3C//DTD XHTML 1.0 Transitional//EN" 
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"
>
<html xmlns="http://www.w3.org/1999/xhtml"
>
<head runat="server"
>
 <title>Form 
	Validation</title>
 </head>
<body>
 <form id="form1" 
	
	runat="server">
 <script language="javascript"
	>
//Write JavaScript Code
</script>
 <div>
 <table >
 <tr><td>Name</td>
 <td><asp:TextBox ID="TextBox1"
 runat="server"
 Width="193px"
	 
			 ></asp:TextBox></td></tr>
 <tr> <td>Email ID</td>
 <td><asp:TextBox ID=
		 "TextBox2"
 runat="server"
 Width="193px" 
 ToolTip="Enter unique Email ID"
 AutoPostBack=
	 "True">
 </asp:TextBox></td></tr>
 <tr><td>Contact Number</td><td>
 <asp:TextBox ID="TextBox3"
	runat="server"
Width="193px" 
 AutoPostBack="True"
 ></asp:TextBox></td></tr>
 <tr><td> Sex</td>
 <td><asp:RadioButton ID="RadioButton1" 
	runat="server" 
	Text="Male" 
	GroupName="a" />
 <asp:RadioButton ID="RadioButton2" 
	runat="server"
	 Text=
	 "Female" GroupName="a"
	 /></td>
 </tr>
 <tr> <td>Enter&nbsp; City</td>
 <td><asp:TextBox ID="TextBox5"
 runat="server"
	 Height="22px"
	 Width=
	 "193px"
	 ></asp:TextBox>
 </td></tr>
 <tr><td>
 Password</td>
 <td><asp:TextBox ID=
	"TextBox6" 
	runat="server"
	 Width="193px"
	 TextMode="Password"
	 >
 </asp:TextBox></td> </tr>
 <tr><td>Re-Enter Password
	</td>
 <td> <asp:TextBox ID="TextBox7"
	 runat="server"
	 TextMode="Password"
	 Width="193px" 
 Height="22px"
			 ></asp:TextBox> </td> </tr>
 <tr><td><asp:Button
ID="Button1"
	 runat="server"
		Text="Submit" 
 onclick="Button1_Click" 
						
	OnClientClick="return validate()
	 " />
 </td></tr></table></div>
 </form>
</body>
</html>

(追記) (追記ここまで)