If you haven't, and it sounds like you might not have, you need to do the Broken Shore questline on at least one character before you can go to Argus.
If you open up the adventure guide (Shift-J by default) the suggested content should hopefully lead you to it, otherwise, go see Khadgar at Krasus' LAnding in Dalaran.
If you have done the Broken Shore questlines, then doing the above should also lead you to a quest in Stormwind that will kick off the argus chain.