Safety.
I have worked and exercised in gyms for over 25 years. The most important thing to remember is Safety First! I have seen many horrific injuries that were completely preventable if safety had been more of a priority. Let us look at a few ways to avoid injury.
Look at the equipment you will be using. Make sure it is in good working order and not damaged. Especially pay attention to cable machines and bands. Cables can often be frayed and waiting to snap on the next user. I personally had a cable machine break in the middle of a chest exercise when I was a teenager and partially tore my triceps. Bands will stretch out over time and develop tears in them. When the tear gets large enough the band will snap under great tension and come back at you. I witnessed someone doing a band row when the band snapped at hit them in the forehead leading to staples and stitches.
When it comes to barbells and dumbbells your technique needs to be sound. Always pay close attention to your form, the strength and heavier weights will come over time. You need to build a solid foundation before you build a house on it. The same is true for your body. Learn the movements increase weights slowly and before you know it you will be strong and injury free.
Do not put clips on the barbell when bench pressing
This is the most single important safety tip I can give you. I have seen so many people trying to bench press too much and fail. If you have clips on the ends of the barbell you are stuck. The only thing you can do at that point is roll the barbell down your midsection or hope someone is nearby to help you. I have personally aided hundreds of people from a bad injury. When the bar gets stuck on your chest it usually will roll onto your neck and that is it. If you do not put clips on the barbell, you can tip the bar and the weights will fall off the side. The best thing is to bench with a spotter or with people around. If you are alone do not try and lift too much. Please be safe.
Always use the safety handles on a squat rack
There are safety handles on squat racks for a reason, use them. If you sustain an injury while squatting or cannot come back up, they will save your life. I have seen numerous times when people went down with a barbell on there back and got injured very badly. Please be careful.