Rails Responsive View

in my application at the moment of creating a user can select ins birth date by choosing a year a month and a day in three different dropdowns, at this moment the user is able to choose a future date as his birth date, what i'm trying to accomplish is a way to check if the user is selecting a valid date and warning him in case the date is invalid. I know i will need to use some javascript but i never used it in rails before. My main problem is how can i access the inputs from the js file. Thanks in advance.

Code that corresponds to the date selection part in the form:

<div class="control-group">
  <%= f.label "Date of Birth (YYYY-MM-DD)", :class => 'control-label' %>
  <div class="controls">
    <%= f.select :year_Of_Birth , options_for_select((Time.now.year - 100)..(Time.zone.now.year )), :include_blank => true  %>
    <%= f.select :month_Of_Birth, 1..12 , :include_blank=> true  %>
    <%= f.select :day_Of_Birth, 1..31 , :include_blank => true  %>
  </div>
</div>