For any Macs made in the past few years you don't need to buy a headset for Ventrilo. You just need a cheap pair of headphones or earbuds. You can use the built-in mic on your Mac.
If using the built-in mic, don't use external speakers, because it will cause an echo. Move your Mac as close to you as is comfortable (that will improve the sound to others). Set your mic input volume fairly high (perhaps 3/4) in System Preferences / Audio. Then set your Outbound volume in Ventrilo per the instructions of others in the Vent room (they know how loud you sound!).
If you already have a USB or bluetooth headset, you can use that too of course. You will sound more clear to others in Ventrilo.
When you go in Ventrilo settings, you will see where to tell it to use the built-in mic, and where to set your Outbound volume.
One last thing: you will be using a key (probably left Shift) to ensure you are only heard when you want to be. You are responsible for anything that comes over the channel. The built-in mic can pick up the voices of those around you.
