You will want to dress, as you would, if you were hired for that job. Actually, you should dress a little better. It is better to over-dress than to under-dress. It shows you are making an effort. Fill out your employment application completely and neatly. Come to the interview clean and well-groomed. Iron your clothing. Show up ten minutes early for your interview. Be prepared for standard interview questions like:
What are your strengths and weaknesses?
Be sure to emphasize your numerous strengths, that employers value, like reliability (good attendance, showing up on time, reliable transportation), honesty/integrity, past experience, education, social skills, etc. Self-reflect on ONE thing that you can improve upon.
Why should I hire you for this job?
What do you think are good/bad qualities of a supervisor?
In a previous job, can you remember a time when you had a disagreement with a supervisor or co-worker? How did you resolve it?
Have reference letters ready, from past jobs or from people who know your character well (a church minister, school teacher, or places where you have volunteered). Have names, addresses, and phone numbers ready for your references. It's a good idea to ask permission from those people, who you are using for a reference, so that they are not caught off guard when they are called, to provide you with a reference.
Have good eye contact, speak clearly and assertively. After the interviewer asks you a question, hesitate a moment to formulate a response in your mind. If it is a tough question, that you weren't prepared for--ask for a minute or so to think about the question. If the question is negative in nature, like "Tell me about your weaknesses." In your response, turn it into a positive. If the question ask about a time when this or that happened, but you cannot think of an example, then answer how you would handle the situation if it came up in the future, instead.
At the end of the interview, be sure to ASK FOR THE JOB! Thank the interviewer for his/her time and for giving you the opportunity to come in and interview. Immediately after the interview, go home and send a thank-you note to the interviewer. This will get your name in front of them again and make a good impression of yourself.
Good Luck!