<form method="post" id='myForm' name='myForm' onsubmit="return checkform(this)" action="/cgi-bin/script/520/9/98">
  <script type='text/javascript'>
//<![CDATA[

      function checkform(form) {
      //validate the terms checkbox
if (!document.myForm.acceptance.checked) { // ! means "not"
    document.getElementById("trmserr").innerHTML = "You must accept the terms and conditions";
    document.getElementById("myForm").acceptance.value = ""; // clear the field
    setTimeout("document.getElementById('myForm').acceptance.focus()", 25); // refocus on it
    return false;
} else {
    document.getElementById("trmserr").innerHTML = "";
}
            
return true;
}

        <span id="trmserr" class='formErrorWarning'></span>
          <p>
            <a class='tooltip' href='#'>
<span class='compulsory'>*</span>
<span class='the_tip'>This is required because, by proceeding, you are accepting these terms and conditions for this purchase. They will be emailed to you along with your order confirmation.</span></a>  <input type="checkbox" id='acceptance' name="acceptance" /> I have read and I accept the terms and conditions.</p>