If you need to add additional fields to Magento’s default contact form, read on! First open the form.phtml file located in your /magento_root/app/design/frontend/YOUR_INTERFACE/YOUR_THEME/template/contacts/form.phtml. So, we are going to add a subject field to this contact form so that customers tell us the subject of their email. Open the file for editing in your favorite editor.
Find this section in the file below (this should be around line 39):
<label for="name"><?php echo Mage::helper('contacts')->__('Name') ?> <span class="required">*</span></label>
<input name="name" id="name" title="<?php echo Mage::helper('contacts')->__('Name') ?>" value="<?php echo $this->htmlEscape($this->helper('contacts')->getUserName()) ?>" class="required-entry input-text" type="text"/>