The oldest IRC network in existence is EFnet. Others include Undernet, Dalnet, and NewNet. In order to connect to any network, you will have to establish a connection to any server on that network. SmIRC comes with a small list of servers, and allows you to add new servers to the list.
When you establish a connection, you will have to provide a nickname, by which you will be known on the network. No two people can have the same nickname. SmIRC lets you enter a second choice for a nickname. If your first choice is used already by somebody else, SmIRC will automatically register your second choice. If it is taken up as well, you will have to manually type in a command to pick another nickname.
Often, your favorite server will be busy, in which case you will have to try another one. Most servers accept connections on multiple ports, so it may be possible to get connected to a different port on the same server, if the usual one is busy. The default IRC port is 6667, but since it is the default port, it is usually the busiest one. If you can't get through on port 6667, try a nearby port. Most IRC servers accept connections on ports 6660 through 6669. SmIRC can be given a port range, to select randomly from.
The IRC network is divided into channels. Each channel has a name, and a conversation topic. Depending on the size of an IRC network, there can be from a hundred, to ten thousand - or more - channels in existence at any time. Once you join a channel, anything you type is seen by everyone else on the same channel, and you see everything they type. It is also possible to send private messages to a nickname, whether or not they are on the same channel as you are. It is also possible to join more than one channel at the same time, if you think you can keep up with them all. Each channel you join is displayed by SmIRC in a separate window.
You can obtain a listing of all channels currently in existence, and join one. Or, you can pick a new name, and create that channel simply by joining a channel that does not yet exist. Once everyone leaves a channel, the channel automatically goes away.
SmIRC keeps a list of all nicknames in a channel, displaying it on the right side of a window. The individual who first creates the channel becomes a channel operator. A channel operator can make anyone else on the same channel to be also a channel operator. A channel operator is just like anybody else on the same channel, except that the operator can forcibly anyone from the channel, and do other things. SmIRC is highly customizable, but in the default configuration, channel operators are listed in red, with a '@' in front of the nickname of the operator.
If all channel operators leave a channel, then nobody else can become one, since the only way somebody can become a channel operator would be if they are made one by an existing channel operator.
A channel can be an invite only channel, which means that nobody can join that channel unless a channel operator issues an invitation to this nickname. It is also possible to issue an invitation to a channel that is not invite only. In this case, anybody - not just a channel operator - can issue the INVITE command. If a channel is not an invite only channel, an INVITE command doesn't really do anything, except cause a message to come up on somebody's screen, inviting them to join the channel.
A channel can be a private, or a secret channel. This means that the channel will not show up in a channel listing. You'll have to know the name of the channel, in order to join it. Usually, a private channel is also an invite only channel, but it doesn't have to be. Usually, the way it works is that a channel is created invite only and private. Then, when a channel operator wants to invite somebody into the channel, the channel operator issues the invitation using the /INVITE command.
The nickname sees the invitation on the screen, which gives the name of the channel, and joins the channel. (Of course, there is no requirement for anybody to join a channel after getting an invitation, unless they want to).
Other ways to control who gets into a channel is to set a channel key (another word for a password). Then, people must specify the same key if they want to join the channel.
A channel can be moderated. On a moderated channel, the channel operator designates which nickname, or nicknames, are allowed to chat. Others are prevented from sending messages, until the channel operator gives them their turn at the plate.
It is possible to send messages into a channel even if you have not joined the channel. Again, a channel operator can set a parameter which will prohibit that.
There are other parameters that can be set on a per-channel, or a per-nickname basis.
Normally, anybody can set the topic for a channel. A topic is basically a title, nothing else, that is displayed by most IRC clients in a channel window, and which is shown in a listing of all existing channels. A channel operator can set a flag which will allow only operators to change a channel topic.
It is possible to give a command to obtain a listing of all nicknames who have joined a channel. You can mark yourself invisible, which means that your nickname will be omitted from the listing. The only way your presence would be known by someone would be if they joined the same channel as you.
There are a few other parameters that are even more obscure. See the MODE command for more information.