When you encrypt a database you usually specify a boot password, which is an alphanumeric string used to generate the encryption key. (You can also specify an encryption key directly.)
The length of the encryption key depends on the algorithm used:
It is a good idea not to use words that would be easily guessed, such as a login name or simple words or numbers. A boot password, like any password, should be a mix of numbers and uppercase and lowercase letters.
You turn on and configure encryption and specify the corresponding boot password on the connection URL for a database when you create it:
jdbc:derby:encryptionDB1;create=true;dataEncryption=true; bootPassword=clo760uds2caPe