When is the right time?

Long story short. I've recently just started a new job. (chipotle) I love my co workers, my manager and their chill environment but I don't like the job itself... I'm not a big people person. And constantly having to talk to strangers is completely out of my comfort zone. It puts a lot of pressure on me, especially when every little thing has to be done a specific way and my manager is watching my every move. He told me I make things awkward when I don't speak enough.. and that my hours there depend on how well I do. Pressure much? But I can't help it. Monday is my official day from being a "trainee" too an employee and I've decided that I don't want the job. When should I tell them?