Returning player is lost (as always).

[Niwashi]

1) What would be the standard "path" to follow in-game?
Not doing quests -> more quests, by "standart path" I mean about changing classes, going somewhere else other than the main path, these things. I know I have to level up Marauder, but I'm not sure when do I stop leveling lancer and go for Marauder.

At least by the time you hit 30, though you could do so sooner if you wish. The requirements to unlock Dragoon are that you have to be at least level 30 as Lancer, at least level 15 as Marauder, complete your level 30 Lancer quest, and have done the Main Scenario up through at least the (level 20) Sylph-management quest. It doesn't matter which order you complete those requirements.

2) I can become whenever class I want? Like, become a Dragoon and then become a Ninja?
I know that I can become whichever 1st job I want (marauder, lancer) but I don't know about these higher classes

You can advance as many of the classes and jobs as you want. My own main character, Y'kayah, is currently a level 50 Bard, level 50 Ninja, and level 37 White Mage, and I'll get all the others eventually. There are players with all 13 combat jobs and all 11 crafting/gathering classes fully leveled. It just means dividing your time and attention between a greater number of goals, so can take a while, but none of the jobs limit what you can do in others.

As Risvertasashi mentioned, it's a good idea to at least try out each of the classes for a bit, see what they're like and whether any of the others interest you enough to continue further.

[Risvertasashi]

My own main character, Y'kayah, is currently a level 50 Bard, level 50 Ninja, and level 37 White Mage, and I'll get all the others eventually.

Sorry to go a little bit off topic but just curious... holding off on Heavensward?

[Niwashi]

Sorry to go a little bit off topic but just curious... holding off on Heavensward?

With so much content at 50 that I haven't done yet, I want some time to finish off ARR content before getting HW. I've actually already bought the expansion, but haven't registered it yet because I want to stay at level 50 until I finish the 50 content. I like being on-level for the content I'm doing and don't want to accidentally outlevel either the old ARR endgame content or the early part of Heavensward content before I get to it.