Guild Banners. The Banner of Cooperation and Battle Standard of Coordination will increase the experience from kills. They both have a 10 minute cooldown, but when you use one, the other is only locked out for 2 minutes. I was very bad about using these, so it might be worth using a macro to drop them on your first attack against an enemy. Keep in mind you must have reputation with your guild before being able to buy these, so new character may have a bit of a wait before picking them up.
Congratulations on reaching Level 120! Now that you have hit Level 120 we recommend looking at our Easy mode page and Talents section to learn how to play at max level. To get yourself prepared for the end-game content ahead, we strongly recommend this Battle for Azeroth Raiding Lexicon/Spreadsheet by Medivh. It is a gold trove of information that will help you when gearing up in Battle for Azeroth and more.
This is also the reason why when I read in WoW forums of people skipping guide sections, due to being too low level I'm not agree and start ranting about it: it'll be a mess syncronizing with the guide and I've seen many people complaining about missing quests or accusing the guide for being innacurate, but not realizing that was their fault since they're skipping and missing a lot of quest chain requirement. Of course this apply if you're following a guide, believe me, it's very frustrating when you remain stuck in a guide section due to the reasons I've mentioned, more than doing grey quests (but they allow to be perfectly in sync with the guide) and goiing here and there to get the pre-requisite quests.
Something to keep in mind as well is that this will make enchanting materials more valuable, since it is likely most players will now simply scrap their boe greens instead of getting them disenchanted. Also redundant dungeon/raid gear can be scrapped rather than traded to the (guild) enchanter in the run. Less stuff gets disenchanted, meaning there will be less ecnhanting materials, meaning their value will rise. All the more reason to DE rather than scrap as an ecnhanter.