You'll only have to access the website and click 'upload file', choose the file to upload, and then, click 'shroten'. That's all.
The file will be hosted and ready to download within seconds (depending on the size of the file and the upload speed).
Cloudapp is a very interesting application for all those users who usually need to work with the same file in different computers. It's also very useful to share smallfiles with many friends, just upload it and send them the link.