The user "myname" (whatever your logonid really is) needs to be placed in the /etc/sudoers file in order to install packages. To edit the file, you have to su to root and type "visudo" (without the quotation marks). That will open your /etc/sudoers file. Scroll through the file, once opened for editing, until you see a section that looks like this:
## Allow root to run any commands anywhere
root ALL=(ALL) ALL
myname ALL=(ALL) ALL
## Allows members of the 'sys' group to run networking, software,
## service management apps and more.
# %sys ALL = NETWORKING, SOFTWARE, SERVICES, STORAGE, DELEGATING, PROCESSES, LOCATE, DRIVERS
## Allows people in group wheel to run all commands
# %wheel ALL=(ALL) ALL
## Same thing without a password
# %wheel ALL=(ALL) NOPASSWD: ALL
See the line that begins with "myname"? Put a line in that location that looks just like it, only substitute your logonid for "myname". Then do a SHIFT - colon (
, and type "wq" (without the question marks) to write the changes and exit visudo.
Now you can do your "sudo apt-get update", if you have installed the Debian tools. For Fedora and other Red Hat derivatives, though, normally you would do a "sudo yum update". Either way, when you use sudo to execute a command with root privilege, you will be asked for your password (the password for your logonid; not for the root password). Enter the password. Your package manager should complete the update.