Q: iMac 2008

Hard to answer without knowing what version of OS X you are running now, but:

Start by checking if you can run Snow Leopard:

Requirements for OS X 10.6 'Snow Leopard'

http://support.apple.com/kb/SP575

(NB: PowerPC applications can still be run in Snow Leopard using Rosetta, but they will not work in later versions of OS X. For detailed information see this user tip:  https://discussions.apple.com/docs/DOC-6271 )

The OS 10.6 Snow Leopard install DVD is still 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 any further updates are required, particularly to iTunes (you will need the latest version in order to synch with mobile devices).

You should now see the App Store icon in iTunes, and you now need to set up your account:

http://support.apple.com/kb/HT4479

You can now upgrade to Mavericks OS 10.9 for free IF you have one of the following Macs, with not less than 2GB of RAM, and at least 8GB of available space on your hard drive:

iMac (Mid-2007 or later)

MacBook (13-inch Aluminum, Late 2008), (13-inch, Early 2009 or later)

MacBook Pro (13-inch, Mid-2009 or later), (15-inch, Mid/Late 2007 or later), (17-inch, Late 2007 or later)

MacBook Air (Late 2008 or later)

Mac Mini (Early 2009 or later)

Mac Pro (Early 2008 or later)

Xserve (Early 2009)

iCloud system requirements:

http://support.apple.com/kb/ht4759

If you cannot run Mavericks you can purchase the code to use to download Lion from the App Store (Lion requires an Intel-based Mac with a Core 2 Duo, i3, i5, i7 or Xeon processor and 2GB of RAM, running the latest version of Snow Leopard):

http://store.apple.com/us/product/D6106Z/A/os-x-lion

or Mountain Lion:

http://store.apple.com/us/product/D6377Z/A/os-x-mountain-lion

Since you do not say what Mac OS X version you have, you can update to Snow Leopard, 10.6.8, by purchasing the installation disk from Apple's on-line store for $20.  From that point you can go to Lion if you have an Intel Core 2 Duo processor or better by purchasing the download from the Apple on-line store for $20, or if you meet the Mountain Lion/Mavericks specs in http://support.apple.com/kb/HT5842 you can go to Mavericks for free from Snow Leopard.