finishing on top of unknown bathroom wall material
 

I removed a big mirror in my bathroom and uncovered this unknown bathroom wall material (photo attached). I'm not sure what it is exactly, but I know it has a glossy finish. Anyhoo, how can I cover or refinish this and have it look decent? I'm hoping to put a framed mirror over the sink and maybe a cabinet to the left of it.

Looks maybe like some sort of panelling that has been painted over, maybe just even hardboard.

yup, found out it's panel board or some FRP stuff. Anyhoo, I want to put in a smaller mirror (42" W x 30" H) and a custom narrow cabinet or shelving unit (22" W x ~30" H x 8" D) to the left of it. This would essentially occupy that whole space (which is 74" W x 30" H). I think I can handle custom making my cabinet/shelving unit and mirror frame and holder, but I don't know much about mounting them properly. How do I mount these items on this wall securely?