Your very old version of OS X, and the version of Safari it can support, are no longer able to cope with today's flash videos and other steamed material from the internet.
You should try to obtain OS 10.5 Leopard from either eBay or Amazon. The the final version of Leopard is 10.5.8 and is the last version of OS X your iMac can support.
Make sure you obtain a black retail disk, and not somebody's grey update disk which will not work on your iMac.
When you have managed to install whatever version of Leopard it might be, you can update it 10.5.8 from here:
http://support.apple.com/downloads/Mac_OS_X_10_5_8_Combo_Update
then use software update to update all the various applications from Apple.