You need to do a few quests to get access to Antorus.
First you need to do the quest “uniting the isles”. This involves you getting at least friendly with the 5 main reps that was at launch (this does not include the wardens rep) as well as doing the quick intro chain to Surumar (as this quest phases Dalaran). Afterwards you will have the quest to assault to broken shore. Once you finish off the intro stuff to the broken shore, you need to do the intro quests for Argus, which starts in Dalaran.
Afterwards simply head to the Antoren wastes and do a quick quest that will blast open the entrance to the raid.