How to set up a debian mirror?

Question

linux

Good day.
One Vasya Pupkin from the video advised not to install software packages when installing debian. As a result, I can’t install them at all after the first login ... I log in as a user, I enter commands (in bash), nothing works, or the command is not known. How to set up downloading packages from a mirror?
UPD: I'll answer my own question.
If you have not installed anything and have not enabled the download of packages from the mirror, at the first start we go as the root
user
: # before cd-rom.
2. Next, in the same place, add the line for the location of the mirror (repository). I chose yandex and entered:

deb http://mirror.yandex.ru/debian/ stable main contrib non-free

3. Save CTRL-O , then press Enter. If it is written that there are no rights, then most likely you are not logged in as the root user.
4. Install the packages we need. In my case, this is KDE:
apt-get install kde-standard