Which allied race are you trying to unlock? I ask because you mentioned Legion, but the War Campaign is Battle for Azeroth.
Regardless, I can at least get you started on the basics.
You will need an appropriately leveled character of the right faction to do the final unlock quest chain, but some of the prerequisites can be unlocked on either faction. Void Elves, Lightforged Draenai, Highmountain Tauren, and Nightborn require a level 45 character for the final unlock. The others require 50.
The two simplest to complete the prerequisites for are Highmountain Tauren and Vulpera. These just require completing the associated questing zone, and can be started at level 10 for Highmountain and 35 for Voldun.
All of the other ones will require unlocking end game systems, the main step of which is unlocking World Quests.
For the Legion, you need to be level 45 and complete the quest “Uniting the Isles” from Khadgar in the Violet Citadel in Legion Dalaran. As it’s your first character through, you’ll need to get to friendly will each of the 5 zone factions to complete the quest. (You can work on the first 4 factions starting at level 10, but Nightfallen (Suramar) requires 45 to start.
I recently posted this quick overveiw for the Void Elves and Lightforged Draenai.
For Nightborn, you basically have to do pretty much everything in Suramar. (The main exceptions are you do not need to do the dungeon quests or the quests to get to X reputation level with the Nightfallen.) Here’s a guide to Suramar.
For BfA, the required level is 50 and you’ll need to get your Heart of Azeroth before you’ll get access to the World Quest unlocks.
- Complete the 3 foothold quests for the War Campaign. (You should be sent to start the War Campaign shortly after finishing the BfA intro quests. Each foothold unlocks at a different level before 50.)
- Get your Heart of Azeroth. The Earthen Guardian NPC will have the quest “A Dying World” to start this. For Alliance, the NPC is near the Boralus Inn. For Horde, they are near the Zuldazar Harbor flightpath. (I’m not sure of the current level requirement for this quest, but there’s not a whole lot of reason to do it before 50.)
- Unlock World Quests. The quest you need to complete for this is “Uniting Kul Tiras” (A)/“Uniting Zandalar” (H). On your first character, you’ll need to do the 3 foothold quests to unlock the boat travel to the 3 zones of the other faction for this quest to be available.
You can work on the main story questing before 50 for both Alliance and Horde. Zandalari and Kul Tirans will require completing all of the leveling zones for the respective faction plus a questline that ties it all together. (Most of that can be done before 50.) They also require the War Campaign, most of which is locked behind the World Quest unlock.