You nick in IRC is the name you go by, and how people get to know you, and how you get to know others. You can set it to most anything you like, with some exceptions for which characters to use.
Some channels require you to have a registered nick. This means that nick has been reserved for you, and you alone. One of these channels is #meta, which is where a lot of people meet and chat.
To register your nick (Once you've picked one), you can do the following:
/msg nickserv register MySuperAwesomePassword email@example.com
Change 'MySuperAwesomePassword' and 'firstname.lastname@example.org' to fit you (ie, a good password, and your email address).
You will get sent a confirmation email, which will have a code in it. You must then message nickserv with this code, to confirm your account.
Once connected to IRC, you can then login using the following:
/msg nickserv identify MySuperAwesomePassword
If you've forgotten your password, you can use the RESETPASS command to send yourself an email with a recovery code.
/msg nickserv help resetpass
If you still know you password and just want to change it, you can use the SET command:
/msg nickserv help set password
Nickserv itself is self-documenting, which means you can interact with it, to get more help.
/msg nickserv HELP