Option one: Tor on Debian Stretch - stable, Debian Buster - testing, or Debian Sid - unstable

If you're using Debian, just run as root:

# apt install tor

Debian provides the LTS version of Tor. Note that this might not always give you the latest stable Tor version, but you will receive important security fixes. To make sure that you're running the latest stable version of Tor, see option two below.

When Tor is installed and running move on to step two of the "Tor on Linux/Unix" instructions.

Option two: Tor on Ubuntu or Debian

Do not use the packages in Ubuntu's universe. In the past they have not reliably been updated. That means you could be missing stability and security fixes.

Raspbian is not Debian. Tor might run fine on the Raspberry Pi 2 / 3 but not the first generation Pi. These packages might be confusingly broken for Raspbian users, since Raspbian called their architecture armhf but Debian already has an armhf. See this post for details.

Admin access: To install Tor you need root privileges. Below all commands that need to be run as root user like apt and dpkg are prepended with '#', while commands to be run as user with '$' resembling the standard prompt in a terminal. To open a root terminal you have several options: sudo su, or sudo -i, or su -i. Note that sudo asks for your user password, while su expects the root password of your system.

apt-transport-tor: To use source lines with https:// in /etc/apt/sources.list the apt-transport-https package is required. Install it with

# apt install apt-transport-https
to enable all package managers using the libapt-pkg library to access metadata and packages available in sources accessible over https (Hypertext Transfer Protocol Secure).

sources.list: You'll need to set up our package repository before you can fetch Tor. First, you need to figure out the name of your distribution. A quick command to run is lsb_release -c or cat /etc/debian_version. If in doubt about your Debian version, check the Debian website. For Ubuntu, ask Wikipedia.

Now Tor is installed and running. Move on to step two of the "Tor on Linux/Unix" instructions.

The DNS name deb.torproject.org is actually a set of independent servers in a DNS round robin configuration. If you for some reason cannot access it you might try to use the name of one of its part instead. Try deb-master.torproject.org, mirror.netcologne.de or tor.mirror.youam.de.

Use Apt over Tor

deb.torproject.org is also served through via an onion service: http://sdscoq7snqtznauu.onion/

To use Apt with Tor, the apt transport needs to be installed:

# apt install apt-transport-tor

Then replace the address in the lines added before with, for example:

# For the stable version.
deb tor://sdscoq7snqtznauu.onion/torproject.org <DISTRIBUTION> main

# For the unstable version.
deb tor://sdscoq7snqtznauu.onion/torproject.org tor-nightly-master-<DISTRIBUTION> main

Now refresh your sources and try if it's still possible to install tor:

# apt update
# apt install tor

See onion.torproject.org for all torproject.org onion addresses.