You need to edit the '/etc/apt/apt.conf ' file or if that file does not exist, you need to add a new file in '/etc/apt/apt.conf.d/01proxy'
Then you need to add the following line to one of these files.
Acquire::http::Proxy "http://[proxy server ip]
:[proxy listening port] ";
You need root level privileges to do this task. After editing the you can run a "apt-get update" command to update the apt database.