How to validate HTML Form Using JavaScript

My Facebook Profile
<html>
<head>
<script type="text/javascript" language="javascript">
function shakeel()
{

var a=document.forms["aaa"]["fname"].value;
if(a==null||a=="")
{
alert("Fisrt Name Must be Enter");
return false;
}
var b=document.forms["aaa"]["lname"].value;
if(b==null||b=="")
{
alert("Last Name Must be Enter");
return false;
}
var c=document.forms["aaa"]["namef"].value;
if(c==null||c=="")
{
alert("Father Name Must be Enter");
return false;
}
var d=document.forms["aaa"]["namem"].value;
if(d==null||d=="")
{
alert("Mother Name Must be Enter");
return false;
}
var h=document.forms["aaa"]["mob"].value;
if(h==null||h=="")
{
alert("Enter the Mobile No.");
return false;
}
var i=document.forms["aaa"]["add"].value;
if(i==null||i=="")
{
alert("Please Fill the Address.");
return false;
}
var j=document.forms["aaa"]["em"].value;
if(j==null||j=="")
{
alert("Please fill the Valid Email id.");
return false;
}
var k=document.forms["aaa"]["emailid"].value;
if(k==null||k=="")
{
alert("Please select the Domain.");
return false;
}
var g=document.forms["aaa"]["course"].value;
if(g==null||g=="")
{
alert("Please Select the Course");
return false;
}
var e=document.forms["aaa"]["age"].value;
if(e==null||e=="")
{
alert("Please Select the Age");
return false;
}
if ( ( aaa.sex[0].checked == false ) && ( aaa.sex[1].checked == false ) )
{
alert ( "Please choose your Gender: Male or Female" );
return false;
}
{
if(document.getElementById('agree').checked)
{
alert('Thank you.! Your Form is Sucessfully Submitted.');
return true;
}
else
{
alert('Terms rejected. Can not proceed....');
return false;
}
}




}
</script>
</head>
<body bgcolor="#CCFF33">

<center><form name="aaa" action="" onSubmit="return shakeel()" method="get">

<center><u><h1>Registration Form</u></h1></center><br>
<table>
<tr>
<td>First Name:</td>
<td><input type="text" name="fname" /></td>
</tr>
<tr>
<td>Last Name:</td>
<td><input type="text" name="lname" /></td>
</tr>
<tr>
<td>Father Name:</td>
<td><input type="text" name="namef" /></td>
</tr>
<tr>
<td>Mother Name:</td>
<td><input type="text" name="namem" /></td>
</tr>
<tr>
<td>Mobile No.</td>
<td><input type="text" name="mob" /></td>
</tr>
<tr>
<td>Address-</td>
<td><input type="text" name="add" /></td>
</tr>
<tr>
<td>Email_Id</td>
<td><input type="text" name="em" />@<select name="emailid">
<option value="">select Domain</option>
<option value="emailid">gmail.com</option>
<option value="emailid">yahoo.com</option>
<option value="emailid">hotmail.com</option>
<option value="emailid">live.com</option>
<option value="emailid">rediffmail.com</option>
<option value="emailid">ymail.com</option>
<option value="emailid">outlook.com</option>
</select></td>
</tr>
<tr>
<td>Course</td>
<td><select name="course">
<option value="">select course</option>
<option value="course">10th</option>
<option value="course">10+2</option>
<option value="course">Gradutaion</option>
<option value="course">PG</option>
<option value="course">MBA</option>
<option value="course">B.Tech</option>
<option value="course">MCA</option>
<option value="course">BCA</option>
<option value="course">BBA</option>
</select></td>
</tr>
<tr>
<td>Age</td>
<td><select name="age">
<option value="">select age</option>
<option value="age">18</option>
<option value="age">19</option>
<option value="age">20</option>
<option value="age">21</option>
<option value="age">22</option>
<option value="age">23</option>
<option value="age">24</option>
<option value="age">25</option>
<option value="age">26</option>
</select></td>
</tr>
<tr>
<td>Sex</td>
<td><input type="radio" name="sex">Male<input type="radio" name="sex">Female</td>
</tr>
<tr>
<td></td>
<td><input type="checkbox" id="agree" /> I accept terms and conditions.</td>
</tr>
<tr>
<td></td>
<td><input type="submit" value="Submit" /><input type="reset" value="Reset"></td>
</tr>

</form></center>

</table></body>
</html>