Speak with Bath'rah the Windwatcher.
Now that you've learned your berserker's stance, you'll want a nice weapon to use it with.
Talk to Bath'rah the Windwatcher, a troll hermit who lives between the Alterac Mountains and the Western Plaguelands. He is a skilled weapon maker and a powerful shaman.
To find Bath'rah, go to the river that flows along Tarren Mill's eastern flank. Follow it north--Bath'rah is camped near ruins that rest along the river's eastern bank.