Jeez, the Injustice fans are such jerks. Guy asks a single question, gets responses back full of telling him to learn himself instead of helping him out. I don't even like this game but I will at least tell him how to play.
For things in the Enivronment, press Light and Medium (Defualt Square and Triangle) or the R1. An Icon should show by your lifebar if you can interact with the environment.
For Clash, press your Meter Burn Button (Normally R2) And forward while the opponent is attacking you. You can only do this when you are in your second health bar and you only get one a match, there should be a yellow bar under your characters name showing if you can do a clash.
A guide you can use to help you learn is provided below.