Are you running Snow Leopard? You need to be to upgrade to Lion. So upgrade to that first. Unfortunately, that means a second purchase from Apple in addition to the $29.99. OS X Lion will set you back.
Make sure compatibility issues with current apps won't be a problem after you upgrade. Many programs and some drivers reportedly have trouble with OS X Lion. There are also known problems with Microsoft Office 2011 for Mac.
Just in general, take a mile-high view and ask yourself if you really want to upgrade in the first place. First movers are first to run into problems. Do you really need any of the new features in Lion? No reason why you can't use Snow Leopard while Apple irons out fixes over coming days, weeks, or months.
One of the biggest changes to Lion is it supports Macs with Intel CPUs only. That means it doesn't support the Rosetta add-in previous OS X versions have--that plug-in allowed apps that worked for the PowerPC chip to work. No more. So do you run Intuit's Quicken? Check updates at Intuit and at other app makers to make sure they didn't make use of the Rosetta translation feature, no longer in OS X Lion.
To determine if your system has an Rosetta-based apps installed, run the Terminal and type this command:
Thanks to BYTE's Ron McCarty for that list tip. OS X Lion certainly is forward-leaning, but it's always a good idea to make sure your system has what it takes before jumping to a new upgrade as feature-packed and significant as this one.