First you need to check that your current Mac meets the System Requirements for Both Snow Leopard and Lion...
Snow Leopard Specs:
If it does... then you need to Purchase and Install Snow Leopard Disc,
And then Purchase and Install Lion...
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/
Also... For Lion... Check the Compatibility of Third Party Applications you may be using...
Once you have SL 10 6 8 installed you can use the App Store to get Lion - Providing your Mac meets the System requirements - .
Will also draw your attention to this link again...
For Lion... Check the Compatibility of Third Party Applications you may be using...