If you did not play the Legion expansion, getting to Argus may be a difficult task as there are quite a few quests you will need to complete before you are able to unlock the zone originally added with patch 7.3.
Players will want to travel to Argus either to complete Antorus, The Burning Throne raid or complete the "You are now Prepared" achievement to unlock Lightforged Draenei and Void Elves as Allied races.
World Of Warcraft: How To Get To Argus In The Shadowlands
To begin unlocking Argus, you will need to complete the introduction quests to the Legion version of Dalaran which will require level 45 as of the Shadowlands.
These are Uniting the Isles, Armies of Legionfall, and Assault on Broken Shore.
If Khadgar is not visible in Legion Dalaran to begin the first quest, he might be in the old Wrath of the Lich King version.
Once these three quests are complete, you should be able to begin The Hand of Fate, from the Violet Citadel in Dalaran.
This quest will lead you into Argus and begin the questline to start unlocking all three islands.
If you wish to complete the Antorus raid, you will need to unlock the island by progressing through the story, which will unlock at Securing a Foothold.
Ultimately if you have friends you are running the raid with you and they have it unlocked, they can summon you using the meeting stone out the front of the raid.
This questline is good to complete anyway if you want to complete the You are now Prepared Achievement for Allied Races but is not required to finish to unlock the raid.