Need help on End Game mechanics.

So I came back after a year of not playing a game and was overwhelmed by the new content and a lot of.. unknown things.
Back a year ago, we had Syrcus Tower raid and were trying to collect alagan tomestones for level 100 weapon, as well as collecting tomestones for equipment. All has changed now a bit, and now that i came back i have no freaking idea where to find everything and how am i supposed to know everything ?

I saw in Ishgard some shop where you can buy some green items level 145~ items where you can put materia in. you can buy them for some 5 yellow things. Now where do i get those things from ?.. Are those green items now all there is, or are there blue items somewhere to be found ?

Basically where am i supposed to find equipment lately ? because mor dhona has everything up to ilvl 120. but where after that ? lvl 120 weapon requires some different sort of allagan tomestone, where to get that from.. ?
i just have no idea where to find everything..

Seeing as you have heavensward, just do the main storyline for gear - it will definitely gear one of your classes on the way to 60 and most likely a secondary (so long as you are choosing rewards that make sense lol).

In terms of guide for mechanics I recommend watching some of the popular youtubers, you can also try some websites - generally a google search of the area you're concerned about will populate your results with 10 different guides.

Green is dungeon equipment, pink is aetherial gear and can have better stats of similar level green gear, blue is rare equipment (your end game stuff), purple is epic (relic items). As you go through the main story you will unlock the next hunt seals and new tome stones, you can of course spend your lower tier tomestones on gear but for your first job to 60 you shouldn't need to and would likely take you a lot more time to gear up if you did.

Some of the many places you could look for guides (Possible Spoilers):

Youtube: MTQ, MrHappy, LightAndMelon
Sites:
http://ffxiv.gamerescape.com/wiki/Category:dungeon
http://www.ffxivguild.com/
http://ffxiv.consolegameswiki.com/wiki/Dungeons

120 is more than sufficient to start you into Heavensward content. In fact, the first few Heavensward quests will literally throw HQ level 115 gear at you, and the game will continue to throw relevant gear at you up to 60, nevermind dungeon drops.

The accessories you are confused about from a vendor are tradeable from Amber, which are rare treasure drops that you can find in the new Levequests. A normal Levequest chest will contain one; a "large-scale" levequest (costing 10 leves) will have a full set of 5 if you get lucky and find a chest.

Thanks for help guys!:)
I checked some info and it seems like i can start getting allagan tomestones of law only when im 60 from roulettes, and to spend them on end game gear i need to get to Idyllshire. I havent gone there yet cos i decided to switch from scholar to astrologian as soon as i landed in Ishgard, so it will take a while to get to 50 and then to 60, good thing is that all my ilvl 120 equipment i will be able to use on AST as well except the weapon which hopefully i will get from following main story quests that are from lvl 50 to 60 :X.

There is so much to do in this game, but i guess i just have to take it one step at a time :)