Americans talk briefly with people they do not know: cashiers, neighbors, people in a line. This is called small talk. It is friendly, short, and not personal.
"How are you?", "How's it going?" and "What's up?" are usually greetings, not real questions. The person does not expect details about your health or problems.
Good answers:
"Good, thanks. How are you?"
"Fine, thank you. And you?"
Always ask back. If you only answer, it can sound cold.
With close friends, you can answer honestly: "Honestly, it's been a hard week."
The weather: "It's so cold today!"
The place you are in: "Is this line for the pharmacy?"
Weekend plans, food, travel, sports, pets, a TV show.
Compliments on things, not bodies: "I like your jacket."
Start: "Hi, I'm Amina. I just moved in on the third floor."
Keep the other person talking: ask open questions like "How long have you lived here?"
End politely: "Well, it was nice talking to you. Have a good day!" or "I'll let you go. See you around."
Americans say "We should get coffee sometime" or "Let's hang out" warmly, but it is not always a real plan. If you want to meet, make it concrete: "Would you like to get coffee on Saturday?"
Most people use first names, even with managers and professors (unless they say "Call me Dr. Smith").
If you do not catch a name, ask: "Sorry, how do you say your name again?" People appreciate it.
You can offer an easier nickname if you like, but you do not have to. Your name is your name.
Answering "How are you?" with a long list of problems to a stranger.
Thinking an American is fake because they are friendly and then do not call. Friendliness and friendship are different here; friendships build slowly.
Asking a stranger about salary, age, weight or religion (see the guide on topics to avoid).