If you haven't set up iCloud on your devices, it's easy to do, but you have to ensure you're using the same Apple ID on all your gadgets. By default, my iPhone was set to the account I use for the App Store, which is different from my iCloud account.
To enable iCloud in Mountain Lion, the latest edition of Apple's Mac OS X, go to System Preferences and log in with the Apple ID you want to use. I choose to enable all of the services here, such as Find My Mac and Photo Stream, but you can just check the ones that you'll use if you prefer (see image below).
Finally, to make sure everything syncs with your iOS device, you'll have to enable the same options. To do this, go to Settings > iCloud on your iPhone or iPad and check that your account is the same as the one on your Mac and that the same services have been enabled.
On iOS 6, you can sync data from apps such as Mail and Calendar, but all of your open tabs will sync over to the new Safari as well if you're using Mountain Lion. This is a really useful feature if you're browsing at home and have to rush out, but want to continue what you were doing online.