Leo4All's website has a good how-to already written out so I'm just going to point you there and tell you to ask questions as you go if you run into any roadblocks. I agree with Dies, make sure you back up anything you don't want lost by placing it on a separate HD because messing with all the partitions when OS X is still new to you might be a little daunting and you could end up redoing everything later on anyways.

Enjoy.

http://leo4all.com/

BTW... you'll find different variations to this how-to because there are different ways of doing it... for example back with Tiger (10.4): http://coolblog.profit42.com/2007/07/03/updated-osx-10410-dual-boot-guide/

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