Need some AJAX/Javascript validation coded to an existing Wordpress sign up form

General information for the website: Restaurant chain
Kind of development: Customization of existing website
Num. of web pages/modules: 1
Description of requirements/features: Hi

I have a Wordpress website I am maintaining for a new client, they have a sign up form which links off to a third party system by submitting various fields:

email (mandatory text field)
first name (mandatory text field)
last name (mandatory text field)
nearest branch (mandatory select dropdown)
birth date (NOT mandatory)

Currently the form only validates against the first field, email address. I need someone who can program the validation against the remaining fields.

This is whats being used to check the EmailAddress field is being completed...

//Checks that a request is being made via AJAX with at least the email set
protected function isValidRequest(){

global $helpers;

if( ! $helpers->isAJAX() ){

return false;

}

return isset( $this->requestData['EmailAddress'] );

}

The work needs to be carried out on our own test server, FTP access will be available but I will need to know all files and changes that have been made in order for it to be put live later on.

Also we need to be careful on testing submissions to the third party, so once work has been completed and we can see the error message appearing for the other mandatory fields we will need to perform some official tests.
Extra notes: