How to get to Drustvar as Horde differs from how those in the Alliance will get there. There are some prerequisite quests you'll have to complete in the Horde.
You'll need to get to Drustvar to complete the Horde War Campaign quests, so you may be stuck if you don't know how to trigger the Drustvar quest. If you're in the Horde, you'll have to complete some quests around Tiragarde Sound before you can sail to Drustvar.
How to Get to Drustvar as Horde in WoW
When you speak to Dread-Admiral Tattersail on the Banshee's Wail, you'll get to go to Drustvar if you're not in the Horde. If you are, she'll instead offer to go to Tiragarde Sound. You'll have to go there and complete the pre-quests if you want to get to Drustvar.
Complete all of the quests on Tiragarde Sound and Nathanos should give you the option of going to Drustvar. If you're doing the Footholds, you'll have to level up to unlock the quests to choose where to go. By the end of the Campaign, you'll have been to all three areas.
For the quest, you can speak to Gallywix on the Banshee's Wail to sail to Drustvar.