support.apple.com/kb/DL1400 install it, if you get kernel panic, reboot into dvd and time machine back, or use osx86.net/view/2010-chameleon_2.1svn_r1684.html to boot directly to lion dvd/usb
You're rather vague in your request; The reply above assumes you're already running OSX version 10.6x and simply want to upgrade to the last version. If you're running an earlier version or the original MS operating system then you're going to need to create a "Hackintosh". There is plenty of information on the web on how to do that but to summarize you'll need to get your hands on an "iboot" CD and the full retail version of OSX 10.6.3 or 10.6.4 (~7GB DVD), once you've got it installed then you can use the above link to upgrade to 10.6.8.