Chances of getting hired by McDonald's as an 18 y.o.

Go for the interview. They'll let you know what shifts are available. You will let them know your availability. You'll compare to see if there is a match.

The more hours you offer to work the more likely you are to get the job. That said, if you thinking working a lot of hours will have a harmful effect on your studies or mental health, don't work them. Make sure it's clear you are applying for a part time position. You don't want them thinking you can consistently work 40 hours a week. When are you going to have time to study? Maybe that's what you want, but at least be mindful of how that will affect others aspects of your life.

I've never heard of rules against high school kids working past 10 pm. In fact, it is often young people, including high schoolers, who work late hours because they don't have children or families to feed, and obviously go to class during the day unless it's the weekend. I'm your age and have worked until 11 pm in my job. Also, you should offer all hours you are willing to work. Do not decrease your availability because you think they don't want high school students working late hours. If they don't want you working, then, fine, but maybe they do.

The interview is not going to be hard as long as you speak clearly and are confident in your answers and ability to work hard and meet the goals of McDonald's. Go on the McDonald's website and see if they have core values or aims and potentially work them into your responses. Do not disrespect your interviewer or act as though you know better. As for your not being outgoing, honestly I don't think it matters. It might affect your relationships with people at work, assuming you get the job but I don't think it will be a huge factor in the hiring process. As long as you're not a nervous wreck, and show respect, you should be fine. You're not applying to be a waiter, and if you are a cashier, your interactions with customers will be minimal. At McDonald's, customers order their food and then go wait at a table or linger somewhere somewhat distant from the register while waiting for their order.

Keep in mind if you don't get the job it's not always your fault. You can be a great candidate, but different stores look for different things. Some stores want people who already have a connection inside the store. Some stores don't want teenagers and prefer hiring people who they think will surely be more mature about the job.