Syntax: REGISTER password

Registers your nickname in the NickServ database.  Once
your nick is registered, you can use the SET and ACCESS
commands to configure your nick's settings as you like
them.  Make sure you remember the password you use when
registering - you'll need it to make changes to your nick
later.  (Note that case does not matter  FIDO, Fido, and fido
are all the same passwords)

Guidelines on choosing passwords:

Passwords should not be easily guessable.  For example,
using your real name as a password is a bad idea.  Using
your nickname as a password is a much worse idea ;) 
Your password needs to be more then 5 characters long.
Finally, the space character cannot be used in passwords.
