You can sign up for iCloud on your iPhone, iPad, or iPod Touch or on your Mac or PC. Here is how:
iOS devices: Make sure your device (iPhone, iPad or iPod) is running iOS 5 or later.
Then turn on iCloud. If you just updated your iOS to the latest version or if you just turn on a brand new device, setup assistant will help you set up iCloud. If you skipped this process previously and now you want to set up iCloud, simply tap Settings > iCloud
Mac: Make sure that you have a Mac with OS X Lion v10.7.2 or later. Then turn on iCloud (System Preferences > iCloud)
PC: You need to download and install iCloud Control Panel for Windows from here. You also need to set up iCloud on your Mac or iPad, iPhone or iPod. After you’ve installed iCloud Control Panel for Windows, navigate to the Windows Start menu, choose Control Panel > Network and Internet > iCloud.