First off no one likes an a**hole. When you say aggessive that could back fire terribly, instead be a teacher. Have some patience. Its important to lead by example. Once they have at least 3 weeks of experience the jitters should have dissipated and that worker should be settled in to their role. Its okay to be firm with your expectations and you do that by checking their work and teaching what the expected outcome should be. Make sure that each task at hand is clear and the biggest thing that breeds success is communication. I hope this helps you in your endeavours.