If you are asking for the Windows 7 Product key, that you'll need to buy from Microsoft; this site does not support or condone piracy, and a moderator may close this thread at any moment.
If you are asking for the security key for a wireless router, if it is your router you set that yourself. If you did, but forgot it, reset the router to defaults and then re-apply security settings. If it is someone else's router, ask them for permission to use their connection. If they say "yes," they will give you the security key.