First- read the book Asterisk- The Future of Telephony. It’s available from O’Rilley or as a free creative commons download from asteriskdocs.org . I recommend reading it, it provides a solid introduction to Asterisk and what it can do.
Any other info you need is on the Voip Wiki at voip-info.org
If you want to use your Macs and headsets to make calls, you will need a piece of software called a softphone. Many are available, search the Wiki.
That said, I highly recommend against a softphone unless you know you are going to be wearing headsets 24/7.
For hardware, you will need a dedicated computer to run Asterisk on. This could be one of the Macs but i don’t know if zaptel cards work with Mac. Otherwise any old $30 PC will do- for 3 lines and a LAN (ie, no compression/encoding) (leave the fax alone) all you need is an old P-200. You will want a Digium TDM400 series interface card for you lines with at least 3 FXO (red) modules. this may help- digium.com/index.php?menu=fxsvfxo
FXO (red module) connects to a phone line, FXS (green module) connects to a PHONE and provides a dialtone. You can buy hardware at voipsupply.com
Hope that helped.