How do I keep users from accessing other users files?
 

Because I was browsing around in KDE and found out that I can view other users files and they can view mine.

If you want privacy between users, either:

1. Don't allow users to belong to the "users" group, e.,g. create users that only belong to their own "group". This is the easiest way, but it is a bit difficult for a system that is already set up.

2. Change the default file creation permissions for each user so the files cannot be read by others. This is not so easy to implement in a system that is already up.

You can also instruct each user in how to restrict particularly sensitive files to be unreadable by anyone but the owner. Like this:

$ chmod 700 *

From the command line, and a similar command can be issued on a file-by-file basis from most file browsers.