Over the next few weeks you are going to see a lot of crap on Facebook, Twitter, etc, disparaging and lamenting the presence of those who are new to the gym or new to health and fitness. Ignore it. Ignore it all. Instead, accept my congratulations and enthusiasm for your decision to make a real and meaningful change in your life, and realize that this may be the most important and impactful choice you have ever made. It isn't easy, but it is worthwhile.
The reason that more seasoned gym-goers tend to scoff and role their eyes this time of the year, is that the enthusiasm of a fresh start is followed by increased gym traffic (and wait times for equipment) plus the inevitability of beginners using machines improperly, practicing poor form, and most of all, not observing proper gym etiquette (both written and unwritten).
To make your transition a little smoother I have put together a BEGINNER'S TOOLBOX on what to know and where to start:
EQUIPMENT - If the gym is busy (and it will be), limit your time on cardio equipment to 30 minutes and your time on equipment such as the squat rack, bench press, etc to 15 minutes. If someone asks to "work in" with you, it is proper protocol to accept this. This means they will do a set while you rest, and vice-versa. If you are using resistance machines, do not rest on the machine in between sets, get up and allow others to use it while you are not. They are all easy to adjust, and so it is no big deal if you need to make a change while sharing it. Finally, when you are done with all and any equipment, wipe it down and put it away. There is nothing more frustrating than having to cleanup after someone else. I'm not your mother or maid, and neither is the staff or exercisers at the gym.
SIGHTS/SOUNDS/SMELLS - Proper gym attire is really up to the individual, however, a good rule of thumb is: no jeans and denim, wear runners or athletic footwear, and if you know you sweat a lot, either wear clothing that soaks it up or bring a towel. "Sounds" may seem like an odd choice, but this is actually pretty paramount in the gym. Listening to music through headphones is good (though most gyms play music over their speakers). Talking on the phone, talking loudly with your gym-buddy, or using an audible timer on your phone is extremely irritating, and listening to music over the speaker of your phone is a non-starter. Additionally, think about the noises YOU are making. Audible exhaling is ok; grunting, groaning, yelling, shouting, and dropping weights is amateurish, ridiculous, and unnecessary. Finally, your scent is important. There are few things more distracting and nauseating at the gym than someone with strong body-odour. DO wear deodorant, DON'T lather on a ton of perfume or cologne (this is also distracting and nauseating). A fresh, light scent, or no scent at all is best.
SPATIAL AWARENESS - This might be the most important item of protocol. The gym will be busy, but it is critical that you realize the need for giving others space during their set. I like to keep a "1 metre rule", that is, each exerciser is entitled to a 1 metre bubble in all directions that I do not penetrate. Lifting weight takes focus, and having somebody bump in to you or come close to you mid-set, breaks that concentration and can hurt them and you quite seriously. Also, if at all possible, try not to walk in front of people when they are watching themselves in the mirror during the set. Often they are looking at their form and this will surely break their concentration as well. Wait until they are done their set to return your weights. Along these lines, it is also extremely bad form to do your exercise right in front of the weight-rack, so that other people cannot access weights or put theirs away. Keep a distance of at least 2 metres from the rack when exercising. This will allow others access and will aid in not breaking your own concentration when someone walks in front of you (and they will). Lastly, if you are going to do some floor work (stretching, skipping, abs, etc), don't put your mat down directly next to someone else. They don't want your breathing, sweat, and body heat invading their space. Give them at least 4 or 5 feet. You will find that the more hot and bothered you are, the more you will want your personal space respected. Give others this same consideration.
*BONUS - Don't be afraid to ask for help. Perhaps the greatest reason that "New Years Resolutioners" are looked upon with ire is that they often do not know what they are doing. They are seen to be wasting both their own time and the others around them who need the equipment they are (ab)using. I absolutely recommend purchasing a session or two with a personal trainer and asking them to show you proper form and foundational exercises like squats and deadlifts, or to find a friend who is knowledgeable and consult their wisdom. Believe me, you are better off asking a question you think is "stupid" than doing something incorrectly that looks stupid and can hurt you.
The final reason why new exercisers in January are given the title, "Resolutioners", is because regular gym-goers expect you to come for a month or two, and then stop when the going gets tough or the rest of your life takes over again. My best advice here is to prove those smug-bastards wrong. Get a workout buddy, book personal training sessions, schedule it in your calendar; do whatever you have to do to get to the gym, but just get there. Because in 365 days, you will be glad you did. Your better future starts today.