This is an example ERC session which shows how to connect to the #emacs channel on Freenode. Another IRC channel on Freenode that may be of interest is #erc, which is a channel where ERC users and developers hang out.
Run M-x erc. Use “irc.freenode.net” as the IRC server, “6667” as the port, and choose a nickname.
Switch to the “irc.freenode.net:6667” buffer, if you're not already there. You will see first some messages about checking for ident, and then a bunch of other messages that describe the current IRC server.
In that buffer, type “/join <SPC> #emacs” and hit RET. Depending on how you've set up ERC, either a new buffer for “#emacs” will be displayed, or a new buffer called “#emacs” will be created in the background. If the latter, switch to the “#emacs” buffer. You will see the channel topic and a list of the people who are currently on the channel.
If you would like to be able to talk with people privately on the Freenode network, you will have to “register” your nickname. To do so, switch to the “irc.freenode.net:6667” buffer and type “/msg NickServ register <password>”, replacing “<password>” with your desired password. It should tell you that the operation was successful.
If you switch back to the “#emacs” buffer, you can type a message, and everyone on the channel will see it.
If you want to talk with someone in private (this should usually not be done for technical help, only for personal questions), type “/query <nick>”, replacing “<nick>” with the nickname of the person you would like to talk to. Depending on how ERC is set up, you will either see a new buffer with the name of the person, or such a buffer will be created in the background and you will have to switch to it. Begin typing messages, and you will be able to have a conversation.
Note that if the other person is not registered, you will not be able to talk with them.