Class tier list:
https://xpoff.com/threads/class-tier-list-for-wsg.72026/
Wouldn't say that there are any active guilds specifially. Maybe someone can correct me on that. 19s on EU is pretty stale at the moment.
For gear, you want your class-appropriate armor to trigger the 5% bonus increase in the stat-template that Blizzard introduced in Legion (no stats on gear matter, no enchants either). After you made sure that you are fully equipped with your class-appropriate armor, you can start focusing on item level. Item level is another factor to the stat-template that will give you extra stats. The higher, the better.
When it comes to weapons, as a melee, you want to equip the slowest possible 2-hander/main-hand. That is simply due to the scaling inside instanced pvp, where the weapon damage will be scaled depending on the speed of the weapon. Like in WoD, gear still get's scaled to the average item level of each respective bracket (basically the item level of heirlooms). So when a low item level weapon gets scaled up, and it has slow speed, the damage will scale with it to compensate for said speed.
As a caster, you want to equip a wand when possible. As of right now, wands count as a 2-hander (staff) in the stat-template. Meaning, it's essentially going to be calculated as staff + offhand equipped.
Now, a lot of stats and weapon speeds may change next patch, since all weapons, as it stands right now on PTR, is looking to get normalized. That means, that every dagger will have the same speed, every axe will have the same speed and so on.
There are also a lot of stat changes in terms of gear in the upcoming patch if you still care for your twinks outside of an instanced PvP environment. I suggest you look up the latest thread by
@Rob for more detailed information on the new changes with the upcoming patch.
But in essence, gear doesn't matter much, and you could practically wear all greys/whites in order to trigger the 5% bonus in the stat-template.