New Tank

[ArianeEwah]

Then even after marking they were attacking other mobs, so I stopped marking.

Marking takes a lot of time, and honestly most people just want to "get it over with". Don't bother with marking except you have a macro for that.
You attack a mob, and the DPS (and healer) have to attack this certain mob. If they don't bother to change their target they chose, let them have it.

You "could be nice", and take the mob from the DPS, but this way they will never learn. Do it like Graiggy

As you are learing tank - holding aggro, taking the lead, set the pace, rotate CDs - the DPS have to learn also. Let them use diversion, they should attack your targeted mob or AoE as you AoE. You are the lead and you decide how to run the dungeon.

I also had a bard once who did a full single target burst on a mob while I was mass pulling. He did it twice and I took the mob from him. Well, and it happened a third time, and it was a big-a** gorilla with semi-tankbusters. I told my healer friend to let him die and I told bard "if he would pull aggro during a pull again, I would let him die again"... he left
And we actually got a good DPS as replacement after this. ¯\_(ツ)_/¯ Happy ending.

Still, try not to stretch run time too far, even through you are new. Pull as many as you are confident you (and your healer) can handle, and try to take risks by pulling even more. Make a good judgement, don't wipe the party, but try to improve. Don't stop learning tank, it's actually quite enjoyable after the first steps.

[Galgarion]

Marking takes a lot of time, and honestly most people just want to "get it over with".

Seconded, thirded-fourthed. Marks are for raids or semi-important special trash like the Chyme of the mountain that can wreck your group if left alone. Otherwise, the time you spend doing it is time people are standing around getting bored when they'd rather be stabbing something in the face.