A very basic lesson in language files

Let's say your module is called My_Module.

Create a php file called my_module.php - note the lower case!

Add this file to the appropriate language directory like,
root/language/english/my_module.php

For each translated version, you will add the same filename to each of the language directories.

The simple presence of that language file will mean that your module will pick it up automatically.

However, what if you need some of those language defines for a block?
Simple, just add this line near the top of your block,
get_lang('My_Module');