How should I resign?

I just started working for WFM a few weeks ago, I was planning on it being a summer job and coming back next summer, I made sure this was communicated during my interview to my TL and store management and they both agreed and said there are options for me as a student. After I asked my trainer about an LOA, she said she would send an email to get more information but that it should be okay and that I should be able to sign one. I came into my next shift and was approached by my TL and told I did not qualify for it and that I would have to resign. Now that the two weeks of when I'd need to leave for uni is approaching, I'm confused as to how I should resign? The date I'm leaving for campus is the 21st of August, so I'm to assume I'd need to submit some form of formal resignation by the 7th, but to whom, and how?