When you buy a dress shirt, the most important thing to know is your size.
It means knowing the length of your arms and the girth of your neck. Start wrapping yourself with measuring tape, and memorize your sizing!

When looking at a dress shirt size, the first number on the size tag refers to the neck size, the second number to the sleeve length.
As an example a shirt marked ”15 – 33” would mean the shirt has a 15 inch neck size and a 33 inch sleeve length.

Neck size: Measure the circumference of your neck at the point where you would normally close the top button of your shirt (just under your Adam’s Apple) placing an index finger between the tape measure and your neck. If the measurement is between two sizes, select the larger size.    Sleeve Length: Keeping your arm straight, have someone measure from the middle of the back of your neck, along your shoulder and down the outside of your arm to the start of your thumb (the first knuckle). If your measurement is between two sizes, select the longer length.

Standard shirts are now frequently seen with average sizes, as 16- 16 1/2. In this example, it means that the shirt fit for people with a neck size of 16 or 16 1/2.
Standard shirts can also be categorized with sizes as small, medium, large, XL, XXL. But pay attention, the sizing can vary between manufacturers.