Only Apple knows the release date -- the latest rumors have it as July 6, maybe July 14 -- but you can prep your Mac now to make the upgrade go faster and more smoothly.
Make sure your Mac can handle Lion
Lion's system requirements are slightly different from Snow Leopard's, so you need to verify that your Mac can run the new operating system.
Select "About This Mac" from the Apple menu, and look at the "Processor" and "Memory" items in the resulting pop-up.
Your Mac must have an Intel Core 2 Duo, Core i3, Core i5, Core i7 or Xeon CPU for a processor.
As for memory, you need 2GB or more.
You can't do much about a processor that won't run Lion, but it's easy and inexpensive to boost memory in a Mac. Crucial, one of the largest RAM sellers, prices a 2GB upgrade for a mid-2008 MacBook (the low-end model came with just 1GB stock) at $30 or a 4GB upgrade for $60.
You must be running Snow Leopard
According to Apple, you can only upgrade to Lion from Mac OS X 10.6, aka Snow Leopard.
For now, Apple hasn't spelled out any direct upgrade path for users running Leopard, or Mac OS X 10.5, so if you're using that 2007 operating system, you'll first need to migrate to Snow Leopard, the OS Apple launched in August 2009.
Apple and others, including Amazon.com, sell a single-license copy of Snow Leopard for $29, or $49 for a five-license Family Pack.
For those still running Mac OS X Tiger and who have a Mac that meets the processor and memory requirements, the only option at the moment is to update to Snow Leopard using the $129 Mac Box Set.
A five-license Mac Box Set sells for $179.
Update to Mac OS X 10.6.8
Having Snow Leopard on your Mac isn't enough for Lion: You need to update the OS to version 10.6.8, which Apple released June 23.