This page explain how you join channels in IRC
There are different ways of joining channels depending on what client you use to access IRC.
A common way is through text commands though. Such a command look like
/join #channelname
and can be written either in a eventual status window or another channel. If it's different in your particular client, you should receive a message about this from your client, helping you to do it right.
To join multiple channels at once, you can enter the channels in a comma-separated list
/join #channel1,#channel2,#channel3
The IRC protocol allows us to join multiple keyed channels as well using the following syntax:
/join #channel1,#channel2,#channel3 key1,key2,key3
The command listed above would join #channel1 with key1, #channel2 with key2 and so on.