Add the following line to your /etc/apt/sources.list:

deb http://download.virtualbox.org/virtualbox/debian trusty contrib

According to your distribution, replace 'yakkety' by 'xenial', 'vivid', 'utopic', 'trusty', 'raring', 'quantal', 'precise', 'lucid', 'jessie', 'wheezy', or 'squeeze'.

(Up to version 3.2 the packages were located in the non-free section. Starting with version 4.0 they are located in the contrib section.)

The Oracle public key for apt-secure can be downloaded

here for Debian 8 ("Jessie") / Ubuntu 16.04 ("Xenial") and later
here for older distributions. 

You can add these keys with

sudo apt-key add oracle_vbox_2016.asc
sudo apt-key add oracle_vbox.asc

or combine downloading and registering:

wget -q https://www.virtualbox.org/download/oracle_vbox_2016.asc -O- | sudo apt-key add -
wget -q https://www.virtualbox.org/download/oracle_vbox.asc -O- | sudo apt-key add -

To install VirtualBox, do

sudo apt-get update
sudo apt-get install virtualbox-5.1