Leaving after first day

Hi all, I onboarded this past week into a new position and my first day was Friday. For context I’m 22 looking to go into Health Informatics. The job and job environment was nothing like I thought it was going to be, and the environment made me extremely uncomfortable. I plan on going back to school for my Masters and I currently have 2 part time jobs which I held while I was completing my Bachelors. I was excited for this role but it really wasn’t anything I thought it was going to be, and the hiring manager who was very friendly doesn’t even work at the same site as me. I want to resign and just let them know I’ll be starting a full time program for school in January so I won’t be able to work full time as planned, but I’m generally super nervous about the interaction and just hesitating although I know I don’t want to work there anymore. I want to leave like a professional, what is the best way to do it? The guilt and thought about tomorrow morning is eating me alive because something like this is unusual for me. I feel so upset that this didn’t work out and just anxious in general. I appreciate any advice I can be given. Sorry if this felt like a rant.