Ableton Live is a cross-platform piece of software - it runs on Mac or PC.
Mac:
Mac OS X 10.4.11 (10.5 or later recommended)
1.25 GHz G4/G5 or faster (Intel Mac recommended)
1 GB RAM (2 GB recommended)
Windows:
Windows XP or Windows Vista
1.5 GHz Pentium 4 or Celeron compatible CPU or faster (multicore CPU recommended)
1 GB RAM (2 GB recommended)
Windows compatible sound card (ASIO driver support recommended)
QuickTime recommended.
You will also need plenty of hard disk space for installing the software (4.6 GB of space to install Suite 8, less for Live 8) and saving Live projects.
Is Ableton Live on its own sufficient to start making music?
Live comes with a variety of instruments and effects (which varies slightly depending on which version you buy - Live 8, Suite 8, Intro etc.). There is more than enough boxed with the software regardless of which version you buy, to get you started. If you want to extend your sonic library, Live supports third party VST and audio unit instruments and effects.