How do I quit a job I just started?

Last summer I took up a new job to pay for college. Overall I seemed to enjoy it, and agreed with my manager that I could come back the following summer to work there again. However, upon coming back, things have changed. The entire staff is different, with more difficult people. New hires are quitting left and... show more Last summer I took up a new job to pay for college. Overall I seemed to enjoy it, and agreed with my manager that I could come back the following summer to work there again. However, upon coming back, things have changed. The entire staff is different, with more difficult people. New hires are quitting left and right, leaving me with the responsibility of picking up there hours when I only agreed to 25-30 a week. At the end of the day, I feel extremely lethargic and depressed. I know it's only the first week. but I'm not sure this is the environment I want to be in for the next 3-4 months. How should I go about leaving?
Update: How do I explain to my boss I need to quit my job that I just started?
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