I'm not a new player myself and I consider myself to be rather knowledgeable in FF14 as I cleared alot of content and tried out a few classes and often talk to people playing different classes and how they do certain things. I try to learn as much as possible about the game and about the small things they put into the game.

So my question to you is, if you had one advice you could give a new player, what would it be?
And as a little side thing: What is the most important thing to know about your main class? Like when to use your cooldowns, positioning, rotations and so on.

My advice would probably be: Take slow steps instead of big ones. The game gives you one tool at a time and time to learn how to use it. Try weaving it into your current knowledge and expand on it.

For classes: I mained paladin for a long time, so my advice is: Don't expect yourself to mitigate everything perfectly. It will take time and experience until you know when to pop cooldowns and when you can take a hit without any. Your healers got your back, even if they get mad at you.

My biggest piece of advice is to actually open your actions and abilities tab and read what it says. I see a lot of people who have no idea what their abilities do. Some just read a guide and memorize what someone wrote online (which is better than nothing I have to admit) but when things don't go according to plan they seem to get lost or just completely panic. Knowing your abilities inside and out will give you an advantage for those times that you need to think on your feet.

Also, some classes/jobs have things like directional attacks and it helps to know them (or look at the tab again to confirm them if you don't remember).

Do your melee attacks in order This is part of what I said before, but for some reason I see a lot of new players go really far into the game spamming one single melee attack without any idea of the fact that they aren't getting things done as quickly as they could. Also, any bonuses (like extra hate on the tank attacks or just the extra damage bonus) only apply when performed in the proper combo order. In all honesty, you probably should have figured this out on your own at level 5 or so, but I have seen some people get really far without realizing this.

To help keep your combos flowing, place them in a manner that you can easily strike them in 1-2-3 order. Case in point, my basic melee combos are circle-square-triangle on my controller. I also recommend doing something similar for directionals (my flank attacks are on horizontal buttons and my rear attacks are on vertical buttons with only a few exceptions).

You'll notice that when you hit the first attack in your combo that the next one lights up like a proc ability. If you miss the attack, it breaks your combo so you have to start over.

You will level up to level 50 too quickly now. It is really easy to miss important sidequests like unlocking your job at 30 and all of the abilities you get every 5 levels through that. Recently I have had two level 45 gladiators pop up in two low level roulettes (one was Stone Vigil and the other was Dzemael Darkhold). Neither of them had unlocked their paladin soulstones nor had either of them put any levels into conjurer. In short, they weren't ready for the dungeon they signed up for (and would have had much nicer gear).

I don't blame them. The 1-50 grind goes too fast and people may very easily miss the side quests because they don't have the downtime to look for them like we did two years ago. Although the other classes can get away with it, tanks in particular have a pretty big responsibility to gear/power up to keep the flow of the dungeon.

It also causes a problem in that we used to get a lot of time to practice our abilities one at a time as we learned them. We had an opportunity to get those rotations into muscle memory and add them one at a time at a good pace. If you think of it this way, levels 1 to 49 were a tutorial for the end game (as 1 to 59 could be seen as the tutorial for the current end game).

Don't be a jackass A little etiquette goes a long way. There was this guy I used to play with who was extremely arrogant, rude, dishonest and so on. He ended up getting a pretty nasty reputation and was blisted by at least half of the server. He was always causing wipes and blaming other people. For example, one time he fell off the boat in Leviathan as a tank and then went on a tirade about how the summoner was so bad and caused the wipe...not the drowning tank who decided not to move during a ringout attack or use Tempered Will or anything like that. GG.

All he ever did was run his mouth and he could never walk the walk. Almost all of his FC members bailed (some even bailed after only one day). The few who remain are people who only play a little every now and then.

This game is built around party content. You won't be able to play any of it if you can't join any parties.
Own your mistakes. Learn from them. Don't crap on someone who messes up because your turn to cause a wipe is coming.

Don't give people crap about their gear. Simply put, not everyone has a bajillion hours to grind something over and over to get one piece of gear nor do they have the uncanny luck to get something on the first few tries. It is much easier with 3.0's token system, but certain things still take weeks of effort and sometimes you just need to upgrade a part with something else for now. I think that many people don't understand that you can still play quite effectively even without "BIS."

BIS is fleeting The game is constantly introducing new gear. Something new pops up every 3 to 6 months. Don't miss out on everything the game has to offer because you are chasing after one thing that is going to be outdated not long after you get it.

Be content with what you have and what you can get. Play like a Buddhist!