Surfing is a sport and as all other disciplines, it has rules that grant everyone the right to enjoy and perform without endangering himself or others.
While surfing is getting more and more popular not to say mainstream, the number of surfers out in the water has dramatically increased while the number of surf spots tends to remain stable. Hence the line-up can get quite busy not to say overcrowded on some beaches at some period of the year or some hours of the day. That's why it is mandatory to know and respect the surf etiquette.
We have compiled here some of the most important rules :

OBSERVE the spot, the waves and other surfers before getting into the water, mostly if it's the first time you come to a break. You'll then understand how the wave works, where you should wait for the wave, how to handle the section and who are the locals, the good surfers and the beginners.

DON'T DROP in on someone else wave. In the water, there are priority rules and even if they may slightly vary depending on the country or local customs, the principle remains the same everywhere. There is a right of way which means that the wave "belongs" to one surfer. In case of a A frame, a wave breaking both on the right and left hands, there are 2 rights of way. Basically the surfer that has the priority or right of way is the one who is closer to the peak point of the breaking wave (the white water), or the furthest out on the line-up or the first one standing. The right of way should also go to the surfer that has been waiting longest.
At Main point for example, which is a right hand wave (breaking from the left to the right when watching the beach or from right to left if you observe the spot from the beach), the surfer who has the priority is the one furthest right seen from the beach. So if you are paddling and there is someone paddling on your left, he has the right of way. If he is already standing, you should not even be paddling.
This is the most important rule as if you drop in, not only you'll spoil someone else wave (right of wave) but also endanger him and yourself AND the other surfers paddling out that will get a hard time figure out how to get out of the way of 2 surfers at the same time.

DON'T SNAKE! That is on the etiquette side (behavior in the water) one of the most important rule and too often not respected. You should not paddle around someone who should have the right of way (because he was waiting longer than you in the right position) to steal his wave. Snaking is one of the rudest attitude one could have in the water and can really ruin the good vibes in the water. Don't paddle out and catch the first wave coming, that is also super rude towards the people that have been waiting their turn.

SHARE THE WAVES, don't monopolize the break! One may be the best surfer out in the water, it doesn't grant him the right to shoot all waves. Wait your turn and let others get their chance of scoring waves.
This principle works along with the don't snake rule.

RESPECT THE LOCALS. They surf this break year round, sometimes everyday, and usually share it happily with visitors as long as they respect the priority rules and the other surfers, including themselves. Earn their respect and they will even help you enjoy more by sometimes giving waves they had the right of way on, sharing advices about how to get your marks on the spot or handle a section, etc.

BE POLITE! When you paddle out say hello and smile to other surfers. Be friendly. respect others even if they don't surf as good as you do.

DON'T MOB surf spots in large numbers. It is always very upsetting to see a group of 10 people coming together in the water, mostly if they don't blend with the rest of the surfers and try to rule the spot.

PADDLE OUT THE RIGHT WAY. Once again, observe the other surfers to see from where they get into the water. Don't bother other surfers by being on their trajectory when you paddle out. If the timing is not good enough for you to cross on the shoulder, paddle for the white water and duck dive. And don't throw your board unless there is no-one coming and no-one behind or around you.

DON'T ENDANGER OTHERS by presuming your skills and taking-off if there are people on the way that you can't easily dodge. One should always be in control of his board and watching other people in the water. If there is someone on your trajectory, modify it or kick-off the wave. Don't take risks, there will be another wave for you. Furthermore, help other surfers in difficulties!

10° DO APOLOGIZE when you break the etiquette. We all have done it once in a while, intentionally or not, what matters is not to make it a habit and to apologize sincerely when you do. Don't be a prick, don't fight.

11° ENJOY... the session, your waves, the fact that you're not at your office, the sun or even the rain, other surfers' waves (with your eyes only) and HAVE FUN, that all what surfing is about.

12° RESPECT THE ENVIRONMENT, the ocean, the beach and their surroundings by not littering or damaging them. Don't collect corals, don't break the sand dunes, don't damage or remove the plants, etc.