Collecting data from a booking form using Google Analytics -- 2

I would like to collect data that is selected and sent through the booking engine, such as checkin and nights (noches), through Google Analytics (using Google Tag Manager if possible). I've would like to create both events and and custom variables in order to store this information. Any suggestion on how to do it would be very much appreciated. Thanks a lot. Here is the code: <label for="nightsField">Noches:</label> <select class="nights" id="nightsField" name="nights"> <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><option value='6'>6</option> <option value='7'>7</option><option value='8'>8</option><option value='9'>9</option> <option value='10'>10</option><option value='11'>11</option> <option value='12'>12</option><option value='13'>13</option> <option value='14'>14</option><option value='15'>15</option> <option value='16'>16</option><option value='17'>17</option> <option value='18'>18</option><option value='19'>19</option> <option value='20'>20</option><option value='21'>21</option> <option value='22'>22</option><option value='23'>23</option> <option value='24'>24</option><option value='25'>25</option> <option value='26'>26</option><option value='27'>27</option> <option value='28'>28</option><option value='29'>29</option> <option value='30'>30</option><option value='31'>31</option> <option value='32'>32</option> </select> </div> <div class="button-group" id="engine-submit-wrapper"> <input type="submit" id="engine-submit-button" class="btn" value="Ver tarifas y reservar"/> </div> <div class="button-group" id="engine-tracking-link-wrapper"> <i class="icon-lock"></i> <a href="[login to view URL]" class="tracking-link" title="Ver/Cancelar reserva">Ver/Cancelar reserva</a> </div> </fieldset> </form>