JOIN

Usage: JOIN <#channel>
The join command lets you enter a channel. This is the first thing you want to do when you connect to a server. You always have to place a # in front of the channel name. Normally the # symbol is considered to be a part of the channel's name. The maximum channel name width is 60 characters.
Examples:
/join #music
Channels are the virtual environments in which people chat. When you are in a channel, you see all of the text sent to it, as does everyone else present. While you can have private conversations with individuals who are in the same channel as you are, or even with people who are elsewhere if you know their nicknames, most chat takes place in channels.
You can be in multiple channels at the same time, but it can become confusing. Some networks may restrict the number of channels you can be in simultaneously. It is generally best to be in a very small number of channels that are moderately active. Java users can also look at the NEWJOIN command.