So you're new to martial arts and don't want to be a douche at your gym. If that's the case, you need to heed common gym etiquette within the martial arts world. These are etiquette you should know and follow in order to keep safe, and also not look like a big douche. These are not hard and fast rules, but they are common unspoken etiquette that the majority of MMA/martial art gyms follow.
Hit as hard as you want to get hit.
Every beginner will undoubtedly struggle with measuring the pace of sparring and their own strength. With that being said, hit as hard as you want to be hit. That is the unwritten rule in gyms worldwide.
If you are a beginner, be mindful of how much strength you are putting on your opponent. I know it is harder said than done as you get so tense when you start sparring. To remedy this, start sparring with someone who is really controlled. Start off light and escalate up when you feel more comfortable. If you want to go hard---that is fine. But just be warned that you will be hit back just as hard. So hit as hard as you wanna be hit.
Don't treat sparring like a street fight
Come into the spar with a technical mindset, not one of a brawler. That's the difference between someone who is new to martial arts versus someone experienced. An experienced practitioner will come into sparring with goals of improving technique and trying out new things. Someone who is new to martial arts comes into sparring expecting a war. Of course there is a time to throw down when sparring, but if you are new, you should be working on your technique when sparring. So be technical and don't swing for the fences. If you are swinging for the fences, chances are you are going to get hurt really bad by your more experienced partner.
Take care of your training partners
Taking care of your partner is essential to maintaining good standing in your gym. Being sure your partner doesn't get hurt training with you isn't just polite, it is the right thing to do. Your training partner spends time and energy training with you so that you can get better. So out of respect, you should in turn treat him with respect and ensure that they don't get hurt training with you.
Of course accidents happen; you can still hurt your partner no matter how careful you are. But not yanking your submissions, not spazzing out, pulling your power a bit, avoiding your partner's injured area, etc can go a long way in establishing goodwill. You don't want to be known as the guy that consistently hurts your training partners. No one will train with you then.
There is no shame in tapping
If you want to keep training injury-free, you should tap often and tap early. This point is somewhat related to the previous point. If you are training with a partner that takes care of you, they will provide you with some time to tap before you break something. This is especially important with leg locks because it is hard to know when the submission is on tight until it is too late. And leg injuries are some of the nastiest injuries to come back from.
There is no shame in tapping. Leave your ego at the door. Don't wait until something breaks to tap. If you feel that there is really no way out of it, then tap.
Come clean and keep your equipment clean
The gym is a cesspool for germs, especially if your gym is lazy about wiping the mats after every practice. Do not add to the cesspool. Come with a clean clothes and make sure your gear doesn't stink. No one wants to roll with someone whose gi smells like crap.
Make sure to clean your equipment after every class. This means putting your clothes in the laundry and wiping down your gear and letting them get some open air. This is really important for clothes and boxing gloves. If they are left moist for too long, the odor becomes unbearable and pretty much irreversible.
Don't show off
Nothing worse than a new guy showing off. Just don't do it. You may be good and athletic, and you may beat a couple of beginners. But what usually happens to show-offs is that one of the really good students at the gym will have enough of you showing off and humble you.