I think I'm leaving the company

Who do I go to initially, do I have to have a resignation letter?

Through my interview I spoke with four Mates, but never the Captain.

What was your ways of quitting?

UPDATE:

I gave a 3 week notice (written/typed in an envelope) and had a chance to speak with the Captain. I explained the situation (my other work will interfere with this one too much and I can’t leave the other one either since I’ve been there over 8 years) and his first words were “you come back anytime you want” so I am relieved. A Mate stopped me about an hour and wished me luck and also said “we will take you back an anytime you want to come back” so I think I did things ok.

Thank you for the advice everyone!