The three most important features to evaluate when choosing shirts are:
With fabric, choose all natural fibers, preferably 100% cotton, but for summers a cotton/linen blend can be incredibly light and comfortable. The higher the thread count, the better the fabric, and the softer the shirt will feel against your skin. Unfortunately, the price also goes up with the thread count.
When selecting a collar, a medium spread with medium point length is the best option. This collar style is appropriate in most environments, and will flatter the face shape of most men. Button downs are too casual; tab collars look silly without a tie and aren't versatile. Larger spread between the points look great, but can be considered too formal in many business environments, particularly here in San Francisco.
Make sure your shirts fit! You'd be amazed how many men don't try on shirts before leaving a store, and then just live with the improperly fitting results. Have the salesperson you're working with measure you. Then try the shirt on. Button the collar and insert your forefinger between your neck and the shirt. There should be at least a quarter inch between your finger and your collar at the button. Less and you'll be uncomfortable. More and your tie won't fit properly. The sleeves of the shirt should reach to the bottom of your wrist, perhaps even a tiny bit into your palm to allow for shrinkage as the shirt is laundered.