If Lion is your future, make sure Snow Leopard is your present
As we mentioned in our what Mac owners need to know after today's WWDC announcements roundup, Snow Leopard (Mac OS X 10.6) will be required for Lion.
If you are using Leopard (10.5) or less, you must first upgrade to Snow Leopard.
Amazon shows it backordered 2-5 weeks, (available from some third parties), but the online Apple Store has it for US$29 plus shipping. Note that if you are running a version of Mac OS X before 10.5, you should buy the $129 Mac Box Set ($99.99 at Amazon), which includes Snow Leopard, iLife '11 and iWork, but that is a legal/moral obligation, not a technical one. The Snow Leopard DVD will work on any supported Mac, regardless of which OS is installed on it (or even on a new drive).
Not sure what version you are running? Go to the Apple menu and select "About This Mac" and look in the window that appears (see image above). If it says anything starting with 10.6, you are running Snow Leopard. Anything less? Time to upgrade.
(If you are running Snow Leopard, make sure you are running at least version 10.6.6 and get the Mac App Store app!)
You may also want to make sure your Mac is compatible. Apple specifies "[y]our Mac must have an Intel Core 2 Duo, Core i3, Core i5, Core i7, or Xeon processor to run Lion." That information is shown in the "Processor" line of the "About Mac" window, shown above.
