OJS is Open Journal Systems,
PKP(Public Knowledge Project) is a multi-university initiative developing (free) open source software and conducting research to improve the quality and reach of scholarly publishing.

How to setup your own journal with PKP? – hosted version recommended

Step 1 – Download the OJS by PK on their official website – They accept donation that could be useful to encourage them to keep maintaining and developping such as free well-know piece of software.
If you want to install OJS on your server or local machine for testing and learning propose. Open Journal Systems can be used on a web server that is serving other functions, and requires very little in the way of system requirements.
On the date of 11 of September 2020, the current version of OJS is the version 3:

System Requirements to install OJS 3
To run the latest release of OJS 3.x, your web server will need:
PHP 7.2 or later with MySQL, MariaDB, or PostgreSQL support
A database server: MySQL/MariaDB 4.1 or later OR PostgreSQL 9.1.5 or later
UNIX-like OS recommended (such as Linux, FreeBSD, Solaris, Mac OS X, etc.)

Download now Current Production Release (June 26, 2020) of OJS 3