I have a Mac. Its a relatively new model, I got it about 2 years ago.
Desktop.
Anywho, I’m looking to buy the Sims 3 Game.
It says one of the system requirement for a Mac. is to have a Mac. OS X 10.5.
Well I downloaded that and it said that it was “completed downloading”. There was a little application-looking thing on my desktop, and I kept clicking it but nothing happened. And I tested it out with an Application on my computer that REQUIRES at 10.5 or higher and it said I didn’t have it But the app is still on my desktop and it says 10.5.
But it hasn’t done anything? How do I make it work! I want my Sims 3 Game!
NO! I WENT ON THW APPLE WEBSITE AND IT SAID “DOWNLOAD” SO I CLICKED THAT< D:<<<
IT SAID THAT I COULD DOWNLOAD IT!

Oh boy.
You do not install an operating system from within an operating system.
Since you downloaded it you have obtained it illegally.
The best thing you can do is purchase OS X 10.5 and follow the installation instructions included with the OS.
Check your “About” menu to see for sure if you have Mac OS X 10.5, and also note if you have a “Power PC” Mac or “Intel” Mac. This second part is the processor, and can’t be changed. Then, check the specific requirements for the software: does it require Mac OS 10.5 “Intel”?
More & more software is being released for “Intel” Macs only (I know, it really sucks). The PowerPC (PPC) mac’s are still perfectly good computers, but certain software — e.g., newer games — may simply never be backported to this older hardware. (Apple is no longer making PowerPC mac’s).