While you might dress casually in the classroom, you should always dress professionally when interviewing for a professional job or internship. Below are a few tips on how to select an appropriate outfit for a job interview. In general, you want your outfit to fade into the background so that your interviewers can focus on what you say, not your appearance. You don't need to spend a lot of money; there are great options at Goodwill and discount stores, and good grooming and a neat appearance are more important than fancy attire.
Research the Company and Understand What Is Expected
These days it can be very complicated to dress for interviews. It can be just as awkward to dress in a suit for a casual workplace as it is to dress too casually for a professional environment. Before going to an interview, ask the person arranging your interview about appropriate interview attire. It is perfectly acceptable to ask. Some companies are very conservative in their dress policies and require business attire, while others are more business casual. Either way, you should always dress at least slightly nicer than the employees. If you cannot find out how employees typically dress, it is safest to dress conservatively. It is better to come to an interview slightly overdressed than underdressed.
Attire if you identify as male (see pictures here)
Professional: Wear a suit, tie, and dress shoes. The suit should be a subdued, solid color (navy and deep gray are best), and shoes should be black or brown and polished. Wear a long sleeve, button-down shirt (either white or another light color that matches your suit), and a subdued tie (small dots or classic stripes work well). Wear black socks so that you will look professional if you cross your ankles.
Business Casual: If the interviewer tells you that you should dress casually, or you are very confident that they prefer a casual style, you may wear a blazer or sports jacket with dress slacks and either a sweater or a button-down shirt. Another option is a dress shirt and tie, but no jacket.
Make sure your jacket and pants don't clash (again, darker colors for blazers work best) and that your shirt or sweater is not too brightly colored or patterned. Wear black or brown dress shoes and socks to match.
Your look must be polished; iron your outfit the night before and make sure your shoes are clean (you may consider getting a shoeshine before the interview).
Attire if you identify as female (see pictures here)
Professional: You have slightly more options for professional interview attire. A suit (either pantsuit or skirt suit) with a button-down shirt or blouse is most appropriate for a professional interview. The suit should be a solid, neutral color, such as navy, dark grey or black.
The shirt or blouse can be any color that matches the suit but is not too bright or loudly patterned. Make sure your blouse is not low cut; if you feel at all concerned that it is too revealing, do not wear it.
Wear neutral colored shoes, either professional flats or heels (no more than 2-3 inches).
Business Casual: If the interviewer tells you that you should dress casually, or you are very confident that they prefer a casual style, you may wear a skirt or slacks with a blouse or button-down shirt and a blazer (if you do not own a blazer, you can wear a neutral cardigan or sweater instead).
The skirt or slacks and blazer should be a solid, neutral color, such as navy, dark grey, or black. Again, make sure your blouse is not too low cut or too brightly patterned.
If you are wearing a skirt, you may want to wear pantyhose, especially if you are interviewing for a very conservative company.
Make sure the look is polished; your clothing should be ironed and your shoes clean (you may consider getting a shoeshine before the interview).
Make sure hair and nails are trimmed, and if you have facial hair it should be shaved or trimmed .
Longer hair is often best in a sleek ponytail or bun, or you may wear hair down if it is washed and trimmed.
All interviewees should avoid colognes and perfume that might be distracting for the interview, and wear deodorant.
Makeup should be limited. This may include a concealer or foundation, a neutral gloss or lipstick, and mascara. Avoid brightly colored eyes.
You want the interviewer to remember you and what you have to offer, so keep it plain and simple. Simple jewelry is acceptable. Any other jewelry or piercings should be removed. If possible cover tattoos unless it is a very casual or creative workplace.
Adapted from About.com