[Typo3-install] How and where do I set encryption key for the backend?

[Kurt Krueckeberg]

I have just installed Typo3. Going to the Backend intial screen, I get a 
security warning.

" Security warning!
- The backend user "admin" with password "password" is still existing
- The encryption key is not set! Set it in 
$TYPO3_CONF_VARS[SYS][encryptionKey]

"It is highly recommended that you change this immediately. "

What exactly do I need to set in $TYPO3_CONF_VARS[SYS][encryptionKey] and in 
what file will I find
$TYPO3_CONF_VARS[SYS][encryptionKey]?

Thanks,
Kurt