In order to join any of these channels, you can use the web client provided by Espernet, use the landing page or any standalone client (which usually provides a better experience). A good standalone IRC client that's available on Windows, MacOS and Linux is Hexchat.
Please make sure to read the rules of the channels you are joining! A copy of them will be in the minecraftirc.net landing pages and in the topic of the channel (shown when you first join).
More information on IRC can be found on Wikipedia.
- #Minecraft - The original channel made by Notch, housing between 500 and 600 users each day. Discussion on this channel is not always centered around Minecraft as the channel serves as a place for Minecraft's community to just talk about whatever they want (as long as it doesn't break any rules). Generally anybody is welcome here granted they follow the channel's rules.
- #Minecrafthelp - The main support channel. Players who seek help with Minecraft are able to ask for support here and discuss any problems they may have.