How to load a jQuery plugin?


I am migrating a website from plain HTML to a Wordpress theme. The website uses the following JQuery plugin in the header files:

<script type="text/javascript" src="/js/ui.selectmenu.js"></script>

<script type="text/javascript">
$(document).ready(function () {

I am struggling with the correct way to include this in my Wordpress theme's header file.

1) How do I load the script using wp_enqueue_script?
2) How do I ensure the script is using jQuery.noConflict mode?

Many thanks.