in unix or linux there are permissions for user and group and root for every file and device(devices in linux are treated as files), and there's also read,write, and execute permissions for every file
the "chmod" command changes permissions--------type man chmod in a terminal and a "man" page will pop up, all commands have a man page which explain the syntax and options for a command
if you want to change the permissions for a file you dont have permission to change or access as a user it must be changed while logged in as root
http://www.tldp.org
go there and start reading, they have "how-to" guides for almost anything, linux is complicated but very very powerful compared to windows
if your wondering why all these permissions exist, its a security feature, linux is a multiuser OS, you dont want regular users to be able to make system changes or access stuff they shouldnt have access to-----never know, you might have hundreds of users logged in at once, silly mistakes by regular users wont crash the system or change it in any way because of these features