Go to any Sushi restaurant and you will be provided with 3 complimentary items to go with your Sushi eating experience: Pickled ginger, Wasabi (More commonly dyed horseradish) and Soya Sauce.
Traditionally called Gari, this is young ginger which has been pickled in sugar and vinegar and cut into thin slices, meant as a palate cleanser. With so many different types of Sushi and fish, you may want to cleanse your palate in between dishes. It also helps with your body's digestion.
Wasabi is a root originally used in the 19th century to kill parasites in the fish. The 'Wasabi' found in restaurants outside of Japan however is generally horseradish which has been dyed green to imitate real Wasabi. Whether you have real Wasabi or dyed horseradish, use it sparingly as it is very hot! Mix a little bit in with your soya sauce to add some bite to your Sushi.
You will find Soya sauce placed on each table or around the Sushi bar in any Sushi restaurant. In most places you will have an option of the traditional Soya sauce (Red lid) or the 'Reduced Sodium' Soya sauce (Green lid), which has less salt. You are meant to pour the sauce in the provided bowls to dip your Sushi into and you should only pour as much as you think you will need, try not to leave a full bowl of sauce as this is unnecessary wastage.