Using WP Loop, where to put in exactly and with what syntax?

Hi there, I tried to use the MS content copier, but this plugin does not suite my needs. Please help me out with the following:

I have a MS installation (domein set up) in which I'd like to offer users to login on one specific subdomein page (e.g. for posting.

Each post must be entered with a category (steered with plugin restricted categories). Based on the category name the post must be copie completely automatically to one or more subdomein blogs named as the category AND also to the main page. As that the complete post is copied (inclusive all categories, mediafiles taxonomies etc.), the post is present in the blog where it is opened, so that is not a problem.

As a non-developerd, tried to figure out several plugin solutions and also studie de WP codex.
- the plugin The Loops could help, but I do not understand where to put the generated shortcode (index.PHP of home.PHP ?) Also, where to define the plugin in (netword or per subdomein?)
- I feel (please convince wright/wrong me in that) that this should be solved with a small PHP script. How should this script look like? In addition; for administration I would like to store this functions in a CORE lib, so it is implemented directly when creatin a new site.

Anxious to hear from any of you,