set apt-get proxy if you need

When I work, I alwasys need to set a configure in apt.conf by using my company's proxy
If you have the same situation, just follow the steps

1. Open /etc/apt/apt.conf
2. type some information into the file

Acquire::Http::Proxy "http://proxy.example:3128";

reference:
http://wiki.debian.org.hk/w/APT