Is this your first interview? If it is your first time going on an interview the same rules apply as if it were your tenth time. What should you wear as an interview suit? Is an interview suit different from a regular business suit?
What you want to be looking for is something classic and conservative. What is trendy or the fad at the moment is not suitable for an interview suit. I would not choose the current super slim looks with narrow lapels, but something more timeless and classic in nature. The suit doesn't have to be a box, but should have a modern comfortable fit. There is no reason you should look like you were squeezed into the garment like a sardine. Conversely, you don't need to look like you are wearing your father's suit, even if he was a snappy dresser.
When recommending the first interview suit I usually go with charcoal grey solid over navy blue solid. The reason being, charcoal is softer than navy. Navy can create harsh contrasts with your shirt and tie. That's perfectly fine, but for the first time being interviewed it's better to have a more neutral tone in your dress. Pull the charcoal grey suit together with a new white dress shirt with button cuffs and choose a tie that has some color, but not too much. I enjoy small printed patterns and have been moving away from some bold stripes in neckwear. Use the tie to show some of your personality. Ties are very personal in nature in terms of pattern and color. I have sold many conservative, plain looking suits to international businessmen and have them choose very wild colorful ties.
If you are a veteran of the interview then the navy blue suit would work well. I would use the same type of combination with the navy blue suit as with the grey suit above. For interviews I always ask for a white shirt to be worn. It is clean and crisp and makes your face light up and stand out since white reflects light. Darker colors, even a sky blue, absorb some of the light spectrum and only reflect one color. This can create a cast of color on your skin. You need to be careful here to know exactly what color types are correct for your skin tone. If you choose wrongly then your face could become washed out or grey looking. In my opinion the white shirt is the way to go with an interview suit.
Another important item is the model or design of the suit itself. It's best to choose a two button single breasted suit. Right now the two button suit is the most popular style selling in stores so they are easy to find. Wearing a three button or double breasted suit would not be acceptable because both are not being readily used in the business world today. Having pleated or flat front pants is a matter of personal style as well. Most suits today are found with flat front trousers. This is what is selling on the retail floor so manufacturers are more inclined to create that type of pant.
This description should give a good mental image of what the interview suit should look like. After you have it tailored and put in on with the new shirt and tie, you should have great confidence and feel good about yourself. A new suit can raise your self-esteem a few bars. Just remember the suit can't get you that new job, but presentation is fifty percent of the battle. Good Luck.