Syntax: REGISTER password [email]
Registers your nickname in the NickServ database.  Once your nick is registered, you can use the SET command and ACCESS command 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 matters!  ANOPE, Anope, and  anope are all different 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 ;) and, in fact, NickServ will not allow it.  Also, short passwords are vulnerable to trial-and-error searches, so you should choose a password at least 5 characters long. Finally, the space character cannot be used in passwords.
The parameter email is optional and will set the email for your nick immediately. However, it may be required on certain networks. Your privacy is respected; this e-mail won't be given to any third-party person.
This command also creates a new group for your nickname, that will allow you to register other nicks later sharing the same configuration, the same set of memos and the same channel privileges. For more information on this feature, type /msg NickServ HELP NickServ.