Pretty open question... Try Skype (
www.skype.com) to start. It's easy to install, set-up and use, goes through firewalls, and you can conference up to 5 people I think. Tell your friends/colleagues to install it too. You will need to talk on your computers to start.
Once you get that working, you should start investigating what SIP is. It's more powerful and more open, but is a little more complex to set up. regards -Ricardo