To get set up for web app development and deployment, you want to get the following software packages installed on both your development machine and your deployment machine (VPS).
Here's the relevent installation instructions / resources. You will need to read these instructions and follow the directions:
For Debian 10, up to date versions Erlang and Elixir are available as packages in a third party repository from Erlang Solutions. The versions of these packages in the stock Debian repository are too old - don't install those versions.
You should also install PostgreSQL. This can be installed normally from the Debian repositories, packages are: postgresql postgresql-client libpq-dev