Due to its age, IRC is as much defined by its culture as by its technical specifications.
I'm going to try to address the points that those don't cover...
IRC doesn't have anything in the way of offline messaging, though people have found workarounds that involve using IRC Bots to relay messages. Some people choose to log into IRC using a command line client on a server. This leaves them free to disconnect while their client continues to log conversations until the next time they attach to the terminal and catch up. This is fairly common. Sometimes several people make use of a single server, with a bouncer like ZNC to relay for all of them.
As a result, conversations on IRC are often asynchronous. Don't expect it to be like a phone call. Not everyone ruthlessly checks the chat window every time someone says something. Not everyone who reads what you say will have something to respond with. Smalltalk like "How is everyone" is unlikely to prompt a response. If you have a question, ask it, and someone will be more likely to offer input. Don't ask to ask.
Making smalltalk... or not
<Raymon> hello everyone * geeknomor has quit (geeknomor) <thefinn93> hia <Igel> yop <ansuz> pewpew <Igel> cjdroid-engima_voip-autodisc-mptcp.apk <Igel> hehe * Igel wanders off <Raymon> how is everyone doing? * ansuz shrugs <Raymon> so so? <ansuz> irc just isn't about the feels for me <ansuz> ;) <ircerr> ^ <Raymon> lolz alright <ircerr> being a friend of a friend network dont mean we are social nor friendly ;) <Raymon> *chuckles* alrighty <ircerr> saying 'how goes it' on irc is about the equiv to walking into a large group of ppl ya dont know and going 'hows the weather?' <ircerr> ppl tend to do that then quit. ppl usually just wait till someone actually asks something or says something that requires a reply <ircerr> irc.. :)
Some IRC networks have services like Nickserv or Chanserv. Nickserv allows you to register your name, and force others to change their name if you log back into a channel and they have been using your nick. Chanserv allows you to regain Operator Status over a channel where you have received such privileges.
Opinions vary about Nickserv. Some find it a convenience, others are offended by the hierarchy that they necessitate. There are a select few who get to mediate disputes over who gets what nick, and that doesn't sit well with some. Different IRC networks have different rules about nicks. EFNet and HypeIRC allow 9 characters. I believe Freenode allows nicks of up to 14 characters.
Some people have been using IRC since its early days, if you stick around and listen long enough you might meet some really interesting people.
You might end up unwittingly talking to a bot. Don't get fooled.
You might end up thinking that someone who doesn't speak your language is a bot. Sometimes that's the case, sometimes it isn't.
The Hyperborian community really revolves around IRC. If you're serious about getting involved, you really should install a native client.
You will not be able to join HypeIRC without such a client. Webchat will work for EFNet, but at this time there are no publicly available HypeIRC portals.