How can i make a 2d sidescroller

Its really all about how you restrict the movement of characters and entities that controls what type of game play you get, like you can only allow certain axis motion and rotation to limit a 3d games movement to only 2d movement. And if you want 2d graphics, you can use several free sprite systems from the asset store, or just create some sprites/animated pngs or whatever, import in unity, set camera to orthographic, and your good to go! If your lazy and have cash, there are nice frameworks for 2d games in the assert store as well.

Problems with 2d graphics might be things like pixel perfect sizing and aspect ratio, gotta be careful with object scale and such. Good luck!

This is a big topic, but if I interpret "how do i switch from 3d to 2d", changing the main camera's Projection from "Perspective" to "Orthographic" is the first step you'll want to take.