How to add pagination to category templates

You’re asking “how to use template tags”. Actually there’s a Template Tags page in the Codex.

Template tags are used within your blog’s Templates to display information dynamically or otherwise customize your blog, providing the tools to make it as individual and interesting as you are

– Source: Codex entry about Template Tags

As you’ve already done, you can write custom Template Tags (basically just php functions) – aside from those provided by core, like wp_head() or the_content().

When you want to have specific functions on specific pages, simply open up your theme folder, choose the right template that fits to this specific request and add your Template Tag. To see what template is used when, just take a look at the Template Hierarchy diagram in Codex.