New Job

I accepted, signed and attended the first day of the new job. Now after the first day it feels like its going to be the same as the old job which I left for my mental health.

Im at a at-will employment state in the US.

How do I tell the employer that I have made a mistake and now dont want to be here?

I just picture myself getting miserable in this new job.

There is no specific reason just that I think I made a mistake.

EDIT: Thanks all after sleeping on it and completing a few days I will stay on for a few months or until I land a State IT job which is were I actually want to end up as an internal IT unstead of an MSP.