Bring your resume.
Dress professionally. Dress properly. Makes you're clean and your dress clothes ironed. Have your hair groomed and have a confident and happy. Make sure your shoulders are squared, back straight.
Talk about how you found out about the job. Make sure you're calm and confident. Have some research, be prepared, and well rehearsed. Stay on-topic and make sure you have questions for the employer. That makes it show that you're interested in the job.
Some questions that they might ask (prepare yourself basically):
How did you hear about this job?
What do you know about this company or field of work?
Tell me about yourself.
What interests you about this job?
Blahblahblah.