You don't need to direct general questions to a specific person. Rather, you should ask in the channel in general. Someone will usually answer you:

Channels generally have a purpose, and people are often joined to many different channels at once. You'll see many of the same people in different channels. However, what might be appropriate in one channel often isn't in another. When you enter a channel, take a look at it's Topic (displayed at the top, or with the /topic command) for clues.

You can find-out more about a particular person by using the /whois command, for example:

In ChatZilla, try right-clicking a user, then User Commands > Who is.

Because of this, people will often check to see if someone is really there before talking to them. By convention, people will use the person's nick and "ping" or "ping?" instead of saying, "are you there?":

...

...

<jim> pong

Often you'll want to say something to a particular person and not the whole channel. This is known alternatively as 'msg' (short for message or messaging), private message or private chat, query, etc. There are a number of ways to do this, the simplest being to use the /query command:

In ChatZilla, right-click a user > user Commands > Open Private Chat.