<form action="https://www.salesforce.com/servlet/servlet.WebToLead" method="POST" onsubmit="return validation(this);">

<input type=hidden name="oid" value="insert_your_oid_here">
<input type=hidden name="retURL" value="http://www.company.com/thanks.html">

<label for="first_name">First Name</label><input id="first_name" maxlength="40" name="first_name" size="20" type="text" /><br>

<label for="last_name">Last Name</label><input id="last_name" maxlength="40" name="last_name" size="20" type="text" /><br>

<label for="email">Email</label><input id="email" maxlength="80" name="email" size="20" type="text" /><br>

<label for="company">Company</label><input id="company" maxlength="40" name="company" size="20" type="text" /><br>

<input type="submit" name="submit">

</form>

function validation(form) {
      
      
if(form.first_name.value == '') {

alert('Please enter your first name');
form.first_name.focus();
return false;
}


if(form.last_name.value == '') {

alert('Please enter your last name');
form.last_name.focus();
return false;
}

if(form.email.value == '') {

alert('Please enter your email address');
form.email.focus();
return false;
}


if(form.company.value == '') {

alert('Please enter your company');
form.company.focus();
return false;
}

return true;
}