You need to register an account with the services robots. To do this, send the NickServ a "register" command. The syntax for this is generally like this:
/msg nickserv register [password] [email]
where [password] is a password you would like to use to identify yourself, and [email] is... well, an email address you can be contacted at (used only for password recovery).
Then, when you connect, you'll see a message like this:
This nickname is registered. Please choose a different nickname, or identify...
You'll then need to send the "identify" command:
/msg nickserv identify [password]
where [password] is the password you set before. You'll then be logged in to the services.
Requiring that you be authenticated to the services is just an anti-spam/anti-abuse measure by the channel, it requires that you go to a little extra effort to join that griefers probably won't.