How to define shortcut keys for an application?

Are there any guidelines to defining shortcut keys for an application. I understand that these shouldn't contradict with OS level short cut keys but there are so many other applications and how can one ensure that a short cut for your application will be unique and not contradict with a short cut key for some other application.

e.g I define Cmd + N as a shortcut key for creating a new post in my application. Now how can I ensure that this shortcut key hasn't been defined by some other application too?