Q: how to know the password

Just quit that under the Wifi icon, & redo it, but...

I forgot a password in my Keychain

http://docs.info.apple.com/article.html?path=Mac/10.4/en/mh1960.html

Open Keychain Access in Utilities, use Keychain First Aid under the Keychain Menu item, then either check the Password under that item, change it, or delete it and start over.

Resetting your keychain in Mac OS X...

If Keychain First Aid finds an issue that it cannot repair, or if you do not know your keychain password, you may need to reset your keychain.

http://support.apple.com/kb/TS1544