Hi,
iOS is for iPads & iPhones, OSX is what you need, 10.5.8 the highest a G5 can run, which is quite old now, but I use t everyday, I might ask what you wish to accomplish or do with it, as it may not be worth it if it cannot do it. 🙂
Leopard requirements/10.5.x...
* Mac computer with an Intel, PowerPC G5, or PowerPC G4 (867MHz or faster) processor
minimum system requirements
* 512MB of memory (I say 1.5GB for PPC at least, 2-3GB minimum for IntelMacs)
* DVD drive for installation
* 9GB of available disk space (I say 30GB at least)
Classic/OS9 Apps no longer supported.
http://bobskelley.com/2015/01/04/9-reasons-leopard-keeps-power-macs-real-in-2015 /
Trouble is Apple no longer sells it, check eBay & such for the Retail version, not the Gray Discs...
http://www.ebay.com/sch/i.html?_nkw=mac+os+x+leopard+retail+10.5