JQuery :: Create A Function In An External File For Validating The Fields Of A Form?

I would like to know how to create a function in an external file for validating the fields of a form. If someone could please provide the code, it'll be real helpful. The form is as follows:

<form name="contactus" action="" method="get" id="form">
Name: <input type="text" name="name" id="name" class="autoName"></br>
Email: <input type="text" name="email" id="email" class="autoEmail"></br>
Phone:<input type="integer" name="phone" id="phone"></br>
Date:   <input type="text" name="date" id="date"></br>
<input type="submit" value="submit" id="submitclick"></form>