Files must be attached to paged that have already been created. Learn to create a wiki page.
To attach a file, click on the Attachments tab at the top of the page where you wish to attach a file. This tab allows you to browse to any file on your computer. You can add a comment about this file (e.g. "Revised copy with consideration to Bob's comments."
File Versioning
If you wish to attach a newer version of the file (e.g. with both Bob and John's comments) you can simply upload the newer file with the same filename (a separate comment can be left for the different version) and this system will do the versioning for you.
Attaching Images
Images may be attached while you are authoring your page via the Insert Image link just above the text box. This opens a small window which allows you to browse to an image file on your computer.