The simplest way to get started is to find the source code of some free games on the net. There were a lot of them on github and similar places. I do not know at what stage they are developing now. Some projects have probably died in the meantime.
Then try to learn how they are made and whether you can use their code for your needs. But you will need at least basic knowledge of PHP programming language and understanding of MySQL database. HTML and JS are not enough because such a game requires a server component.

If you build something, call us for beta test!