Hi Marina, welcome to the steep but worthwhile learning curve for ezpublish ;-)
Files are a content class within ezpublish. Im assuming you want something similar to the new file creation/adding facility within the admin interface but on the standard user interface instead.
What you need to do is:
* Make sure you have the roles/policies set up to allow the user(s) to create new file objects.
* Create an override template to actually bring up the default edit page for a file (look at template overrides, you may want to add a folder and override that with the correct template to add a file) If you arent sure what templates you want, start by turning on template debugging, so when you add a file through the admin interface you can see which templates it uses (and hence which ones you need to override) * Modify the copies of the default templates (copy the templates that you want to the appropriate directory and modify them)
Hope that helps. If you havent done the tutorial, I strongly recommend that you go through it, as it some parts that are directly related to what you are trying to achieve.