If you're not a programmer, don't worry! All the main Elgg tools are already adapted to use categories, and a growing number of third party Elgg tools use it too.
This plugin uses views and events hooks to allow you to easily add site-wide categories across Elgg tools.
This is a two-line addition to any plugin.
In your edit/create form:
echo elgg_view('input/categories', $vars);
In your object view:
echo elgg_view('output/categories', $vars);
Note that in both cases, $vars['entity'] MUST be populated with
the entity the categories apply to, if it exists.