To get more information about your computer, go to the Apple in the upper left corner of any window, then "About This Mac", then "More Info..." Copy and paste the information here, but omit the serial number and Hardware UUID (if present).
Mac OS X: System requirements for the current version (Lion) - http://www.apple.com/macosx/specs.html
Lion requires you have Snow Leopard first, unless you get a USB installer and wipe your drive.
Snow Leopard 10.6 Technical Requirements - http://support.apple.com/kb/SP575 - note by K Shaffer - http://discussions.apple.com/message.jspa?messageID=12921514
A Mac OS X 10.6 Snow Leopard Application Compatibility List - http://snowleopard.wikidot.com/
I would recommend just going to Snow Leopard as an optimum solution. If you're running older software you may find too much of it refuses to work under Lion. Lion will need more resources. SL is relatively up to date, much cheaper than Tiger or Leopard and easier to find, and will run newer software but also older. The only real reason for Lion is if you want full iCloud capability.
I guess alternatively you could just get a replacement disc for your computer (it should have come with discs). Call Apple Customer Support 1-800-767-2775 (telephone numbers for other countries - http://support.apple.com/kb/HE57). Provide the serial number and specifications of the Mac. They may be able to provide a replacement set of discs for a fee. These discs will be for whatever OS was installed on the computer originally, so if you have an upgraded version you will need to obtain the discs some other way.
