If you don't have a free iCloud account, or if you're just starting out with it, will give you an overview of how to set up a new iCloud account. To store documents in the cloud--no matter which application puts its files there--you also need to activate the setting in the pane in System Preferences, as well as in the Settings of any iOS device you plan to use (to do so, select . Once you've done this, any iCloud-compatible app can store files in iCloud.
For now, only a limited number of applications can store files in iCloud. By I mean documents that you create, not data that an application such as Calendar stores in the cloud. On the Mac, many of Apple's apps do support iCloud, including Preview, TextEdit, the iWork '09 suite (, , and ), and .
Third-party apps that store documents in the cloud include text editors such as , , and ; the PDF editor ; the graphics editor ; and some others. At this point, compatible programs can produce files in Microsoft Office formats, but Microsoft Office itself doesn't support iCloud.