Like Jet said, it would be helpful to know your set-up, which computer you have.
Basically though, to voice chat with another computer, both computers have to be connected (as through the internet) and running programs with compatible protocols. Unless you have an iSight, your best cross-platform program to use is SKYPE - a freeware package that runs on MacOsX, Linux and Windows boxes.
Once you have the connection, using the mic is as simple as talking to your Mac (that is if the mic is built in, like JetwingX says). You can control the gain (volume) in the Sound System Preference. The mini has no built in mic, but the rest of the consumer Macs (and the powerbooks) do. The mini requires a USB solution, like the Griffin iMic.