Q: how do i install an os onto an imac 2008?

Didn't your iMac came with an operating system? Very strange. Anyway, you have two options:

1. If the iMac didn't come with discs, call Apple to get replacement DVDs > http://support.apple.com/kb/HE57 They will send you a DVD with Mac OS X and another DVD with iLife applications.

2. Buy Mac OS X Snow Leopard > http://store.apple.com/us/product/MC573/mac-os-x-106-snow-leopard If you get replacement DVDs, you will get Mac OS X Leopard, an outdated version, but if you get Snow Leopard, you will have a later version and you will be able to upgrade to Lion or Mountain Lion if you want.

To install Mac OS X, insert the DVD and hold the C key while your computer is starting. Then, supposing that your hard drive is ready, follow the steps on the screen to install Mac OS X

Insert the computer's original disks or a newer retail package, restart with the C key held down, and follow the onscreen directions.

(81773)

If they were already configured with that computer, they will be detected automatically. If not, you won't be able to start holding the Option key because the computer won't detect the keyboard. In this case, you will need a wired keyboard only to install Mac OS X

In that case, you have to decide if you want the original DVDs with Mac OS X Leopard or if you want Snow Leopard. I recommend you to get Snow Leopard

Note that the Snow Leopard DVD doesn't contain iLife or the Apple Hardware Test, and that installing the Mac App Store versions of the iLife applications requires buying a further OS upgrade. Getting iDVD or iWeb absolutely requires the computer's original disks or a retail iLife package.

(81774)

That's a good solution. However, you have to put your iMac in Target Disk Mode in order to do it, and unfortunately, if you didn't set up your wireless keyboard before, you need a keyboard connected to the iMac. Also, if you already purchased Mountain Lion for another Mac, you can install it for free on your iMac

You are doing it correctly, but Mountain Lion requires 2 GB of memory. If you haven't got enough memory, buy more memory in OWC or Crucial > http://eshop.macsales.com/shop/apple/memory/iMac

If you need help installing memory, see > http://support.apple.com/kb/HT1423?viewlocale=en_US