If you're going to a gym and you're not sure of the etiquette, let me hit you with some to live by. And then some bits of advice that I think could be helpful.
Rule #1 - Put your equipment away
When you've finished your reps and set, put the kit away. Even if the person before you didn't. And especially if it's something heavy, it's not fair to expect someone else to clean up after you
Rule #2 - Heavier weights go on the bottom
I personally want all the weights to be in the correct order, and spend time doing that, but it's not so important. But, do make sure heavier weights are closest to the bottom, lightest on top (so you can pick up the heavier weight safely, and also if a weight were to drop from a height, you'd rather it was the lighter one!)
Rule #3 - Wipe down your area
Even if you didn't notice you sweat, wipe down the equipment you've touched or sat on, and wipe down any visible sweat that may have fallen on the floor.
Rule #4 - Don't hog one space/equipment
If there is no-one around, you do you! But if you've been on one bit for more than 15 mins and you notice that someone else looking at you a lot, the chances are they want the same bit of kit as you. Ask if they want to share, so you can work while they rest and vice versa. Or, be flexible in your routine and come back later.
Similarly, if you want a bit of kit, don't be afraid to politely ask how long they have left or if you can share.
Rule #5 - Don't give out unsolicited advice
If you see someone that looks like they need help, of course there is nothing wrong with asking if they'd like any help or advice from you. Their answer may be no, in which case, walk away. But don't assume they need your advice and give it without asking. Similarly, feel free to ignore and politely decline when someone might give you some.
Rule #6 - Listen to your own music privately
If the gym music isn't for you, wear headphones. Don't blast your own music out for everyone to hear. Same goes for phone conversations, probably step outside to take them.
And then some bits from me that aren't rules, just good advice!
Advice #1 - Don't be afraid to ask for help
If you don't know how to use something, ask. Most gyms should have a gym instructor who is there to help you. They can often also set you a plan. Which leads onto:
Advice #2 - Go with a plan
You'll get better success if you follow a structured plan, and you'll also make more efficient use of your time. A lot of gyms will let you get a plan for free (your success, means you are more likely to renew your membership, so they want to help!)
Advice #3 - Turn your phone onto Do Not Disturb/Flight Mode
Exercise is so good for your mental health as well as physical. Let this be a time you don't get distracted by your phone. You'll feel so much better for it.
Advice #4 - Don't wear your muddy shoes if you can
If you have no choice, that's obviously fine. But if you wear your muddy trainers, especially on the cardio machines or you're going to have a bigger clean up job after (and remember rule #3, wipe down your own space, so it's really for your benefit too)
Advice #5 - Don't be afraid to move things
If you need a step, and a set of dumbbells and maybe you're going to use a circuit with some cardio, feel free to move bits around the gym. As long as it's safe, you're not taking up so much space that you're in the way of others, and you put your stuff away after, make the gym your own.
Advice #6 - Try not to be too chatty
I'm a chatty catty, I love talking to people, but just remember to be considerate of other peoples (and your own) time. Most people only have a certain amount of time for the gym, so try not to talk for too long, as some people are too polite to end the conversation. Again, vice versa, if someone is chatting too much with you, just politely let them know it's been a great chat, but you need to get back to your workout
Advice #7 - Believe that you belong
The gym in general, but particularly the weight section can instil do much fear in so many people, men and women. It can be an intimidating experience, especially when you're new. But know that it doesn't matter your gender, what you look like or where you are in your fitness journey, you belong there.
This again is where getting help from a gym instructor an be useful, as they can show you what to do, so that you will have confidence to know what you're doing to then be there. And also why following a plan can also help boost your confidence in knowing what you need to do.
Hope this helps,
Happy workouts!