depends on the type of job and your past experience but i will try to keep it general.
it is a no-brainer to be dressed well. there is no such thing as overdressed or overqualified. be well groomed, no facial hair, no cologne. Bring a resume, and it sounds like what he is asking for a cover letter.
you cover letter (and interview) need to present yourself as a competent problem solver, and as someone who has had relevant enough past employment experience to qualify you for the current job. you need to tell a story in your interview in a sense. everything you say, you need to be able to back up. if you say you are loyal, or a hard worker, or enthusiastic, or a quick learner, or good problem solver, you need to be able to say WHY you are that thing.
"I consider one of my strong points to be a leader because at my last job I took the initiative to apply myself in a leadership position for a project that resulted in the creation of a new product that helped the company in this and this way." "I am very customer oriented. For example, i went out of my way to make sure that customers were always pleased in my role as a whatever, and my customer service ratings were always above 90% and I was recognized as employee of the month in march 2012."
if there is a job description, you need to write your cover letter in a way that describes yourself as someone who can do all of the roles in the description if that makes any sense. read the job description, or if there isn't one, think about what the job entails, and craft your letter accordingly. Think about the ideal candidate for the job, and match your experience WITH your willingness to LEARN and GROW in the company and you will be golden. think beforehand about your strengths and weaknesses. come up with 3 of your strengths and 1 weakness, but make sure it's not a real weakness. something like, "i am a bit of a perfectionist." or "i tend to get very emotional about my work because i care so much about it" or some other happy horseshit, but don't like and say "nope. don't have any."
another few basic pointers. sit still during the interview. plant your feet on the ground and don't bounce around. keep your hands away from your face and mouth and the back of your neck. body language is very important. you must be composed and confident. sit up straight and look at the interviewer in the eyes, but not the whole fucking time. dont' say AHM and UMM. if you have something in front of you, don't fiddle with it. put the pen down unless you are writing something. ask for a business card, when you can expect to hear back and who you can follow up with. send a thank you email within 24 hours.
lastly, research the company and ASK QUESTIONS when they give you the chance to. i repeat, do not decline the opportunity to ask questions, and use the time wisely. ask something you really want to know and it will reflect well.
tl,dr: shower, shave. back up every statement you make with anecdotes and facts. sit up straight, don't say UM and ask questions when given the chance. ask about the next steps/process.