The ChatZilla window opens where you will see the word *Client* in the middle of the page. At the bottom are links to a few of the most popular Networks. If your Network is among them just click on the link and ChatZilla will attach to it and automatically add it your Preferences > Startup.
To configure your chosen networks and channels so they automatically connect, select Preferences > Startup and scroll down to the Auto-connect URLs window, select the Network and click Edit. Now add #your-channel to the end, click Apply then Ok.
If your network is not listed, go to the top menu bar and choose Preferences > Startup, scroll down to the Auto-connect URLs window, click Add, and type in your network. For example, irc://your.network, click Apply then Ok.
If you know the channel you want to connect to automatically enter irc://your.network/#your-channel. Don't forget to click the Apply and Ok buttons.
Finally, add few nicks by selecting Lists > Add, typing in a nick and clicking Ok. Two or three nicks are recommended in case someone else is using your first choice.