Apple finally opened to the public service iCloud. Until now iCloud service is only available for developers account. Now, anyone who have an Apple ID to log in on the website iCloud. But still you can not use it.
The reason is because it takes IOS 5 is installed on the iPhone, iPad and iPod Touch and Mac 10.7.2 version before you can start using the service iCloud. But all of that, both IOS 5 and Mac 10.7.2, have not been released by Apple. So you can do now is visit the website iCloud and try entering your Apple ID and password, only to get information that is needed IOS 5 device or Mac.
But it seems you only need to be patient a few hours before the update IOS 5 out and you can upgrade your Apple device.