How do I send someone a private message?
On just about any IRC client, use /msg nickname message. If that doesn't work use /privmsg nickname message.

How do I open an extended private chat?
Try /query nickname. On most clients this will activate a more convenient way to chat to a person, usually opening up a separate window.

What is DCC chat? How do I start one?
DCC means direct client-to-client. Once you set up a DCC chat messages pass straight from you to the recipient, bypassing the IRC network.
To start one type /dcc chat nickname. Even if you become disconnected from IRC you can continue any DCC chats that you have open

DCC chat doesn't open a window on my client. How do I use it?
Use the command /msg =nickname message (just like normal messaging with = prefixed to the nickname).

How do I send a file to someone?
Type /dcc send nickname filename.
In most clients, if you leave off the filename, it will open a dialog box which will allow you to choose a file. Choose the file and click on the "send" button.

I can't receive a file someone is sending to me.
This can happen for a number of reasons.
Your IRC client may be blocking the type of file. For example, all .exe files are blocked by default by mIRC. For help on how to resolve this problem, see the client help channel for your IRC client. (eg: #mirc). If you cannot find help there, ask in #help.
AustNet may be blocking the file because it matches the filename of a virus file. AustNet blocks these files for your safety. If you are positive that the file is not a virus file, then get the person sending the file to you to rename it.
You could have the person on ctcp ignore. Ask for help in the help channel for your client or #help.
The dcc capabilities of your client software may be broken. Some problems may be fixed (consult your friendly help channel) or your client may just not be capable of file transfer.

For more help on chatting, ask online in #help or contact [email protected].

The usual reason is that you're using mIRC and you have an incorrect IP address specified. Go to the Setup menu and open the Local tab. Change all the options so that the Local Host box has a check mark inside it (the IP Address box will grey out) inside the On connect, always get: box.
Then make sure the Lookup method is set to Server. AustNet's IRC server software sends your own IP address to mIRC so it can configure DCC for you.

Other reasons why DCC may not work include:
You're on a network that's not set up to allow DCC.
You are using a SOCKS Firewall.
You use SLIRP or another PPP/SLIP-over-shell program which doesn't allow DCC.
There's not much you can do about this.