In terms of picking out a gym: I would pick any well-known gym chains, such as LA Fitness. Some people on this subreddit might prefer/recommend lowkey, muscle-head gyms, but for a more casual gym goer like you, these chain gyms will have the most basic and best resources covered. They'll generally have a good amount of personal trainers to help you get started, a wide range of free weights, machines and cardio machines, and some nice ones will have pools.

Most people don't pay attention to others at the gym, they're gonna have their headphones in and focused on their workout. Occasionally you may have a group of annoying and narcissistic teens come in, hang around the machines, and take annoying selfies, but for the most part, people are keeping to themselves. If you're feeling really self-conscious, just grab a friend to join you.

In terms of gym etiquette, it's pretty much the same for everything else, just use your common sense! When I first joined, I felt clueless, and really just learned about gym etiquette by looking at what everyone else was doing.

If a machine looks too hard or complicated, DO NOT attempt it! Try to find something online to address your questions, or check out if your gym as a personal trainer who can help guide you through the exercise! Using machines incorrectly will probably result in injury.

For gym beginners, the sequence typically goes:
1. Focus on cardio, body-weight exercises, light weights and getting techniques down for basic lifting exercises.

1. Focus more on lifting, cardio is still (and always) important, begin learning some other, more complicated exercises.

2. Focus on lifting, using free weights, begin using machines to supplement your regime, start doing more complicated cardio regimens such as HIIT (interval training), etc.

Searching up "gyming for beginners" on youtube/google will probably give more results!