PostgreSQL 9.6 should be available on Debian stable (Jessie) from "main" area. Install it using apt: `apt install postgresql-9.6`. Make sure that older versions are not installed since Debian allows multiple versions to coexist but still runs only one version.
You must install elixir 1.4+ from elixir-lang.org, because Debian repos only have 1.3.x version. You will need to add apt repo to sources.list(.d) and import GPG key. Follow instructions here: https://elixir-lang.org/install.html#unix-and-unix-like (See "Ubuntu or Debian 7"). This should be valid until Debian updates elixir in their repositories. Package you want is named `elixir`, so install it using `apt install elixir`
Elixir will also require `make` and probably other related software for building dependencies - in case you don't have them, get them via `apt install build-essential`
NodeJS is available as `nodejs` package on Debian. `apt install nodejs`. Debian stable has 4.8.x version. If that does not work, use nodesource's repo https://github.com/nodesource/distributions#deb - version 5.x confirmed to work.
* You most likely don't want having some application accessing database as a superuser, so you should create separate user for Pleroma. Right now it must be done manually (issue #27).
* The common and convenient way for adding HTTPS is by using Nginx as a reverse proxy. You can look at example Nginx configuration in `installation/pleroma.nginx`. If you need TLS/SSL certificates for HTTPS, you can look get some for free with letsencrypt: https://letsencrypt.org/
On Debian you can use `certbot` package and command to manage letsencrypt certificates.
* [Not tested with system reboot yet!] You'll also want to set up Pleroma to be run as a systemd service. Example .service file can be found in `installation/pleroma.service` you can put it in `/etc/systemd/system/`.
By default, it listens on port 4000 (TCP), so you can access it on http://localhost:4000/ (if you it on same machine). In case of an error it will restart automatically.