Depth Of Field is going to be your friend when trying to photograph reptiles, especially long ones like snakes. It's amazing what you can do by manipulating aperture size.
You also want to concentrate on shutter speeds and exposure values to get images that will be very pleasing to the eye. Your manual should have sections dedicated to the explanation and use of A and S modes.
Lastly, do yourself a favor and familiarize yourself with focus points. When the eyes are in focus the chances of a more satisfactory photo are greatly improved.
As an example, notice the depth of field causes the rear portion of this Garter Snake to be somewhat blurry, however the crisp focus on the eye makes it a pleasing photo altogether.
