The Suit (From a guy who has read read fashion magazines has seen runway shows)
1. Getting the right suit: Firstly, suits are not cheap. Even on sale, you can expect to pay around $150 and up. However, suits do last a long time so think of them as an investment. There are different styles, different fits. Pick the one you fill most comfortable in as well as the appropriate one for the occasion.
2. "Button or not to button" : When walking/standing, button all the buttons minus the last one. (There are certain styles of suits where you need to button all of the buttons). When sitting down, you unbutton the top button (the suit is completely unbutton). Again, there are exceptions depending on the suit you are wearing.
3. The tie... or the bow tie(For those Dr. Who fans): Make sure the tie matches the shirt. Make sure it's crisp (no wrinkles) and that it's length is appropriate. The width of the tie: This depends on your style and the style of your suit. I would keep it safe and go for the normal width instead of the modern skinny ties. The bow tie: One: you MUST be able to know how to put one on. I have heard of people wearing a bow tie and getting asked to demonstrate how to put one on. Two: You better have the personality for wearing a bow tie.
4. LASTLY, DO NOT WEAR A STUPID CLIP ON!!!
Does all of this matter? Yes and No. It depends on the school and the region.