Where to place hooks in Drupal?

Please,

I can't use hooks in Drupal, because I don't know where to place them. I tried to place in template开发者_运维问答.php, unsuccessfull.

This is the hook which I want to use: http://api.drupal.org/api/function/hook_form_FORM_ID_alter/6

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You would call this hook from within a custom module. For example, let's say you wrote a custom module, named bookmark. That module would contain a file named, bookmark.module. From within bookmark.module, you would call bookmark_form_FORM_ID_alter($form, $formstate)