Validation (checking ) of checkbox selection on submit of Form

<form  name=f1>
<input type=checkbox name=c1 id=c1 value=Python> Python
<input type=checkbox name=c2 id=c2 value=PHP> PHP
<input type=checkbox name=c3 id=c3 value=JavaScript> JavaScript

<input type=button name=b1 id=b1 value=Submit onClick=my_check()>
<br></form>

<div id=d1>Output here </div>

var my_c = document.f1.c1.checked;// true if checked, false otherwise

var my_c = document.getElementById('c1');

<script type="text/javascript"> 
function validate(form) { 
// Checking if at least one period button is selected. Or not. 
if (!document.form1.sex[0].checked && !document.form1.sex[1].checked){ 

alert("Please Select Sex"); 
return false;}


if(!document.form1.agree.checked){alert("Please check the terms and conditions");
 return false; } 


return true;
}
</script>

<table border='0' width='50%' cellspacing='0' cellpadding='0' >
<form name=form1 method=post action=action_page.php onsubmit='return validate(this)'>
<input type=hidden name=todo value=post>


<tr bgcolor='#ffffff'><td align=center  ><font face='verdana' size='2'><b>gender</b>
<input type=radio name=gender value='male'>Male </font><input type=radio name=gender value='female'>
<font face='verdana' size='2'>Female</font></td></tr>

<tr><td align=center  bgcolor='#f1f1f1'><font face='verdana' size='2'>
<input type=checkbox name=agree value='yes'>I agree to terms and conditions  </td></tr>

<tr bgcolor='#ffffff'><td align=center  >
<input type=submit value=Submit> <input type=reset value=Reset></td></tr>

</table></form>