An expansion tank may or may not help with a water hammer problem, but if you have a closed system you need some type of thermal expansion control. Since your house is new I'm surprised you don't already have either a tank or pressure relief valve. Since you would probably be able to see a tank, look for a small relief valve either teed off the cold water at the water heater of under one of the vanities or kitchen sink. It would have a line running to the outside of the house. Many plumbers use these instead of tanks because the new home market is so competitive and tanks cost more.
If you decide to install tank, the water heater is probably the easiest location. If possible, install the tank with the opening facing down to minimize sediment build up inside the tank. If you do have a pressure relief valve now and are installing a tank, this valve should be adjustable so you can just also just set it at about 125 psi and forget it.
If you install a tank and still have a problem with water hammer, Sioux Chief makes some mini hammer arrestors which connect to a standard fixture stop and are very easy to install. http://www.siouxchief.com/Frm_MS.cfm