Check that your current Mac meets the System Requirements for Snow Leopard...
Snow Leopard Specs:
If it does... then you need to Purchase the Snow Leopard Disc and Install it.
Would Also Recommend that you do a Bootable Clone Backup of your current Hard Drive Before attempting any Major Upgrade...
By far the easiest way to make a Backup, is to use something like
or CCC http://www.bombich.com/