How do I add jQuery to specific node form? I'm using an Omega sub-theme and I want to do this the correct (best practice) way. 

Basically I'd like to add some script so that selects a value in a select box based on a previously selected value in another select box. I have the code, just need to plug it in to my theme now. 

However, I only want it to load on node creation forms of node type "X".

Any help greatly appreciated, thank you!
Install the JS injector module. Then go to admin/config/development/js-injector. Add a new rule. Fill the JS code field with your script.
In the Pages fieldset select the Pages on which this PHP code returns TRUE (experts only) option and enter the following code:
&lt;?php
if (arg(0) == 'node' &amp;&amp; arg(1) == 'add' &amp;&amp; arg(2) == 'content_type') {
 return true;
}
return false;
?&gt;

Replace content_type with your content type name.
This is a theme independent solution. Another way is adding your script in hook_preprocess_page in template.php file of your theme.