First, I hope that you know that you shouldn't change module files directly, but to add hook function to your module and change that "hook" at beginning of function name with your module machine name.
Second, you have to clear all the caches so Drupal will re-scan your module and figure out that there is new hook function and start using it.
So, you have to create your own module first:
Don't be scared - It's just a folder with an info file describing your module and module file it self (in minimal case).
Then, if your module is called "anna" you should create a function inside your module file and name it:
And clear the cache - after that Drupal should start calling your hook function.