If you already have a high level character or a friend willing to give you gold then you can purchase Heirloom items. These items are bound to account and are accessible through the Heirlooms tab using Shift+P. The benefits of these Heirloom items is that they scale with your level keeping your gear up to date without you having to worry about upgrades, and they provide a significant XP bonus.
Some quests in my guide are marked as "SKIP" and colored in red. These quests are simply either too hard to solo or not worth the XP/time and are skipped. My guide will only list SKIPPED quests if the quest is a direct follow up after completing a quest, not one that you have to click the NPC again to get it. If you hover over the skipped quests, it will give info on why it is skipped in the guide (unless that info is already listed directly in the guide text).
The Heart of Azeroth is an artifact given to the player character from the world soul via Magni Bronzebeard. It is a necklace that can use the power of Azerite, the lifeblood of the embryonic titan Azeroth (See Plot, below). Azerite is similar in function to Legion's artifact power, in that it is gathered normally from playing the game and is used to upgrade an artifact; likewise, the Heart is similar in function to Legion's artifact weapons by having the capacity to be constantly upgraded. Unlike artifact weapons, however, the Heart is shared with all of the player's specializations and can empower all eligible pieces worn simultaneously.
If you’re jumping back in for Battle for Azeroth, we recommend checking out the big “what to do first” guide from Wowhead. You’re also going to want to brush up on specific advice for your class—both Icy Veins and Wowhead have incredibly detailed guides for all of the game’s combinations of classes and specializations. Make sure you go in and figure out all the new (or changed) talents you’ll be using, and start to memorize a typical combat rotation so you don’t die nearly as much in game’s latest incarnation.
Tiragarde Sound and Stormsong Valley are both fairly similar zones, but Stormsong is a little longer, meaning that you might not even have to finish it to hit 120. Tiragarde also comes with a bit of a warning. There are plenty of side quests to do in the zone after you finish the story quests. Don’t do them. It’s not worth the time or the experience you’ll get.