If you're on a network that connects to the internet through a proxyserver, you can't use apt-get.
To fix this use export http_proxy=http://username:password@proxyserver:port/ and
Username and password is not needed if the proxy does not use authentication.
You need to do this in each shell or terminal. To avoid that, you need to add these lines in your /etc/bash.bashrc file. Other commands might use this too. The second option is to only configure apt to use a proxy. Edit your /etc/apt/apt.conf like this: Acquire::http://Proxy "http://proxyserver:port";