I used to use mIRC years ago but I refuse to pay for it now. These days (on OSX, Linux and Windows) I use Xchat. (or Xchat2 on Windows, Xchat Azure on Mac).
[Like someone mentioned above there's a pay version of xchat but if you search xchat2 there's also a free one out there.]
It's pretty simple to set up. I can put together a tutorial for whatever OS you're using but it's not a lot of work to get it going.
Choose a Nick
connect to freenode (as that's the server our channel is on)
then once you're connected type in the text input box "/join #top_hat_sec"
There's a lot more to it if you want to get in depth like registering your nick with /nickserv so that someone can't use your nick. Some channels require a registered nick also (#asterisk is one). But the above is really all you need to get started.