If i were to make an app on a website where the user can use the app without downloading anything (like those miniclip games or like runescape) what type of coding language would i need for this or what language would be most recommended? Thank you

Miniclip games are done with flash, runescape is done with java. Nowadays you can do similar things with javascript and html5 canvas. You can code the server in Node for example.

What type of game are you trying to develop? 2d/3d, platformer, FPS, RPG, MMO ?

The back-end can be literally any back-end language that doesn't matter. (Node as mentioned is a good option if you're familiar with JavaScript, but you could use pretty much any language/framework)
The front-end is the tricky part and depends on what type of graphics you're trying to achieve and what type of game you're developing. There are a large number of HTML5 frameworks and there are also ways to compile many games (like unity games) so they run in the browser.

p5js is an amazing graphics/drawing related library for JavaScript. You can use it right in your browser window.