If you want to try IRC out without having to download a client (though you'll want a client soon enough), use freenode's webchat.
Connecting, joining, and operating
If the link doesn't work once you've installed a client, open your client and connect via the command line:
Once you're in, you'll probably want to register your nickname (which you can change with
/ns help will tell you what you need to know about registration (registration on the server is unrelated to the registration of your IRC client)
IRC will display your hostname (effectively your IP address) and a "real name"; request a hostmask if you don't want your hostname to be visible, and change your "real name" in your client's preferences if you don't like the default.