Web App - How do they work - Specifically on the sites given

I'm a new developer starting out with web dev work. Issue is, I don't know what exactly to start with. I decided to do a simple (As simple as I can) web app. I'm thinking of the following websites:

• http://slither.io/

• http://agar.io/

• http://wings.io/

• http://vertix.io/

lots of io... Anyways, I want to know what these websites used, or what I can use to accomplish what they did.

Now to avoid being too broad..

Exactly what I want

• I would like knowledge on the basic set up it would take

• I would like recommendations on the basic languages that I should begin learning.

• I would like a ball park range of how difficult this could be

*Also, if this is way too broad of a question, sorry about that. I'm pretty new to this.