apt-get: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On Debian possible default full paths are:

/usr/bin/apt-get - from package: apt

You may install the required package with command apt-get install {package}

Error in other languages:
apt-get: bevel nie gevind nie
apt-get: comandă negăsită
apt-get: comando não encontrado
apt-get: commande introuvable
apt-get: command not found
apt-get: käsku ei ole
apt-get: không tìm thấy lệnh
apt-get: komanda nerasta
apt-get: Kommando nicht gefunden.
apt-get: kommandot finns inte
apt-get: komut yok
apt-get: nie znaleziono polecenia
apt-get: níor aimsíodh an t-ordú
apt-get: no se encontró la orden
apt-get: no s'ha trobat l'ordre
apt-get: opdracht niet gevonden
apt-get: parancs nem található
apt-get: perintah tidak ditemukan
apt-get: príkaz nenájdený
apt-get: příkaz nenalezen
apt-get: команда не найдена
apt-get: командата не е открита
apt-get: 命令找不到
apt-get: コマンドが見つかりません