You can join and leave channels with the /join and /leave commands. Remember these commands need to have the "#" character at the start of the channel name:

/join #newbies

There may be several conversations going on in a channel, sometimes called "threads" of conversation, and each person in a channel may be conducting more than one conversation. Once you join, by watching the conversation for a minute you will be able to understand which users are talking with which other users, and where the threads are.

You can leave a channel with the /leave command, or simply by closing the window, after which you can join some other channel.

/leave #irchelp

If you are the first person to join a channel, that means you create it and thereby become the channel operator.