Since you played a lot of GW2, in FFXIV it is important to remember that you have a role. Unlike GW2, you have to go as a tank, DPS, or healer. You can't be everything at once in FFXIV. Sure you can switch classes, but then you are put into a certain role.

In terms of people, I have never met anyone that is outright rude. If anything they only harass you by game mechanics, like abusing the vote kick system. But then you can report them. Happened to me recently and a few minutes after my guild mate reported it, a GM was talking to her about the issue.

Since most people are generally nice, you will be fine in the lower dungeons. Even if you fail to get aggro, it is not likely your party will die. If people see you struggling as a tank they will likely give tips. You can even say when you enter a dungeon "I am new at tanking so sorry if I mess up."

Also make sure to pay attention to your aggro meter and enemy list. The aggro meter shows below each party member's class icon. The number represents the order of who has aggro and the bar shows how close the are to moving up a number.

The enemy list has different color icons next to the enemy name. You want them to be red circles.

I often like to pull all enemies. I will do a flash after I pull them and then focus on using my aggro combo on one enemy. However when my skills are cooling down I switch between enemies just to see if I still have enough aggro. If I start losing one, like a party member is #2 and bar is almost full, I will use my aggro combo on that enemy or aggro skill until it is good again, while still switching between enemies while on CD.

If an enemy suddenly pulls away, I just use provoke on them to get them back.

This is what I have been doing on my tanks which are level 35 right now. By the time you reach 50 and get into the harder stuff where if you don't hold aggro, you are fucked....you should have it down pretty well.

I think it is good to have one of each class at least. I would say a tank, a melee DPS, and a healer. Melee DPS gets you to learn flanking, running from AOEs, etc. Healer teaches you to heal and make choices quickly, but also how to do ranged DPS. Tank teaches you to watch the aggro meters and positioning.