10.4.11 is Tiger.
If yours is a PPC based Mac (a G4 or G5) you can upgrade to Leopard 10.5.8 but no further. Leopard is no longer supported by or available from Apple so you will need to buy a vlack retail install disk from eBay or Amazon, and they cost around $200.
If yours is an Intel based Mac you can bypass Leopard and go straight to Snow Leopard 10.6.8:
Start by checking if you can run Snow Leopard:
Requirements for OS X 10.6 'Snow Leopard'
http://support.apple.com/kb/SP575
The OS 10.6 Snow Leopard install DVD is available for $19.99 from the Apple Store:
http://store.apple.com/us/product/MC573/mac-os-x-106-snow-leopard
and in the UK:
http://store.apple.com/uk/product/MC573/mac-os-x-106-snow-leopard
When you have installed it, run Software Update to download and install the latest updates for Snow Leopard to bring it up to 10.6.8, or download the combo update from here:
http://support.apple.com/kb/DL1399
Check via Software Update whether further updates are required.
You can determine what kind of iMac you have by clicking the Apple icon top left of your finder window, then on About this Mac.