Shirts are actually a very important part of fitting a suit. Although only neckline and cuffs are exposed, if shirts are not fitted properly, no matter how beautiful suit is, it will look weird.
When choosing a shirt, first of all, you need to look at shape of collar.
The shirt collar shape is an element that is easy to overlook. In fact, there are more shirt collar styles than suit collars, and there are over a dozen variations. Different collar shapes suit different occasions, but formal shirts are generally divided into following three types.
One, square collar 
The square collar is also a classic basic cut with a square neckline. If collar is narrow, it is called a small square collar. Pair a slim square-neck shirt with black suit pants for a sharp and classy look.
Second, Windsor Collar
Although collar is also pointed, distance between two points of collar is usually a bit far, even a bit like a line. This collar style is a formal and very old-fashioned collar style, which is very suitable for business meetings and other occasions, and is very popular with political and business leaders.
Three, button-down collar 
The shape of collar with buttons is almost same as shape of square collar, except that there are two buttons at tip of collar so that collar can be buttoned directly on garment. The button collar is an American style collar. It is said that collar fluttered when gentlemen played polo, so it is fixed with two buttons, which looks more elegant.
Different faces and shirts have different collar shapes. SUITISM here suggests that you can match them to occasion you need to attend, or opt for a more classic style that won't get dated no matter how you wear it.