The website is pretty much wrapped around the game itself, meaning it reuses a lot of its assets. To keep things simple, the stable and development versions of the game are also included in this repository, but they'll each require a Node.js server.
Website itself is built using HTML, CSS, Javascript, PHP and MySQL. It needs to run on a server, a local one will do just fine.
You'll have to install an Apache HTTP Server, like AMPPS or operating system specific ones, like WAMP, MAMP or LAMP.
If you're not very keen on using git from the CLI, I recomment using Git Cola for cloning the project and creating patches.
You can create a clone of the project by using git from CLI or with the use of a tool, like the one recommended above.
Create a folder called AncientBeast inside the root of your development server and browse it, the naming is important.
https://github.com/FreezingMoon/AncientBeast.git
Access your phpMyAdmin, create a database for the project and import database.sql file located in the project's root.
You'll have to configure config.php.in file from project root directory, update the database info and save it as config.php. That file will not be tracked so it will be completely ignored from your commits, making things easier for you. In case you're using a different path and/or name for the project's root, make sure you update $site_root to reflect that, or you'll end up not seeing any images on your local website.
You are free to browse existing issues and comment on the ones that you want to take a swing at in order to make sure the issue is still relevant and that nobody else is working on it. You can also drop by the project's Discord server and mention your skills and that you're interested in helping out; you'll be assigned to a specific issue. When you're done, simply create a Pull Request, note that you might need to pull in from master repository before doing so. Your patches will get reviewed and tested, if there are issues with them you'll receive feedback in order to make corrections, otherwise your pull request will get approved and merged into the master branch and you'll getting you credited for your work.
Coding is not the only thing required in order to make this project as awesome as possible, see the How to Contribute guide.