The only thing to do is upgrade your operating system version to 10.5 or higher, as your computer permits.
Start with getting 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). Compare it to the requirements for each system version below, and we can also provide guidance once we know what computer you have.
Leopard 10.5 Technical Requirements - http://support.apple.com/kb/HT3759
Snow Leopard 10.6 Technical Requirements - http://support.apple.com/kb/SP575
OS X Lion Technical Specifications - http://www.apple.com/macosx/specs.html
I have exactly the same problem.
My computer seems to match the Leopard 10.5 system requirements as follows:
Machine Name: iMac G5
Machine Model: PowerMac8,2
CPU Type: PowerPC G5 (3.0)
Number Of CPUs: 1
CPU Speed: 1.8 GHz
L2 Cache (per CPU): 512 KB
Memory: 512 MB
Bus Speed: 600 MHz
Boot ROM Version: 5.2.5f1
What do I do next to upgrade the system from OS X 10.4.11?