What's "shape" mean here? Is this used as a verb here?

What's a "sidecar"?

A "sidecar" is "a small, one-wheeled vehicle attached to the side of a motorcycle to hold a passenger". So when he says "sidecar" he is metaphorically referring to "the person that someone is having an affair with".

Isn't the word "you" a second-person pronoun? Is it alright to use it with plural forms like "are?"

That's right, "you" is a second-person pronoun, but it's actually incorrect to say "is" with "you!" As a general rule, use "are" if you use the second-person pronoun or plurals. Ex: A: Are you okay? B: I am fine, thank you. Ex: You are going to be a great doctor one day. Ex: She is going to be a great doctor one day. Ex: They are so good at cooking.

What's "sorority" mean? Does this generally imply something when used in this kind of context?

A "sorority" is a typical North American female society in college or university. But you're right. It does imply something. There's a kind of negative stereotype about "sorority" girls: they love to party and drink, they like material objects, they're naive, and they only really care about themselves and the other people in the sorority. They're supposedly very cliquey. So I wouldn't say it's a positive thing that he's calling her a "sorority girl." Ex: I would never have thought that you were in a sorority. You don't seem like the cheerleader type. Ex: Do you want to join a sorority next year?

What does "Here are" mean?

You can use "here are" (or here is) whenever you are giving something to someone, or showing something to them. In this case he is "giving" us some tips for better sleep. Ex: Here is the money you lent me. Ex: Here are some pictures of us when we were little.

What does “trendsetter” mean?

A trendsetter is a person or business that leads the way in a popular idea, especially fashion. Ex: Tyra Banks is a trendsetter since she is a famous fashion model. Ex: Superhero movies are a trendsetter around the world right now.