How to prevent form action in case of validation error?

I have implemented js to check for the valid fields in the form.In case of any improper/blank input,the error message pops up as expected.But i'm unable to prevent it to move to the servlet which handles DB coectivity and eventually, blank or improper data gets saved in the table.Can anybody please suggest how to solve this issue?

    <form name="form" action="servlet/Register" method="post" onsubmit="post">
                                                           <div id="errorBox"></div>
                                                            <input type="text" name="Name" value="" placeholder="First Name"  class="input_name" >
                                                            <input type="text" name="LastName" value="" placeholder="Last Name" class="input_name" >


                                                          <div id="email_form">
                                                            <input type="text" name="Email" value=""  placeholder="Your Email" class="input_email">
                                                          </div>
                                                          <div id="Re_email_form">
                                                            <input type="text" name="enterEmail" value=""  placeholder="Re-enter Email" class="input_Re_email">
                                                          </div>
                                                          <div id="password_form">
                                                            <input type="password" name="Password" value=""  placeholder="New Password" class="input_password">
                                                          </div>
                                                          <!--birthday details start-->
                                                          <div>
                                                            <h2 class="birthday_title">Birthday</h2>
                                                          </div>
                                                          <div>
                                                            <select name="birthday_month" >
                                                              <option value="" selected >Month</option>
                                                              <option value="1">Jan</option>
                                                              <option value="2">Feb</option>
                                                              <option value="3">Mar</option>
                                                              <option value="4">Apr</option>
                                                              <option value="5">May</option>
                                                            </select>
                                                            &nbsp;&nbsp;
                                                            <select name="birthday_day" >
                                                              <option value="" selected>Day</option>
                                                              <option value="1">1</option>
                                                              <option value="2">2</option>
                                                              <option value="3">3</option>
                                                              <option value="4">4</option>
                                                              <option value="5">5</option>
                                                            </select>
                                                            &nbsp;&nbsp;
                                                            <select name="birthday_year">
                                                              <option value="" selected>Year</option>
                                                              <option value="2013">2013</option>
                                                              <option value="2012">2012</option>
                                                              <option value="2011">2011</option>
                                                              <option value="2010">2010</option>
                                                              <option value="2009">2009</option>
                                                            </select>
                                                          </div>
                                                          <!--birthday details ends-->
                                                          <div id="radio_button">
                                                            <input type="radio" name="radiobutton" value="Female">
                                                            <label >Female</label>
                                                            &nbsp;&nbsp;&nbsp;
                                                            <input type="radio" name="radiobutton" value="Male">
                                                            <label >Male</label>
                                                          </div>
                                                           <div>
                                                            <input type="submit" id="sign_user" onClick="Submit()"></p>
                                                          </div>
                                                         </form>