If you can get your hands on some fresh wasabi (and you'll probably have to go online to do so), you should prepare it by grating it with a metal grater. You can mix in a little water to form it into a paste, as necessary. The finished product should be light green.
Don't forget the sushi-ku! One of the required elements of successful sushi, a signature part of Japanese food, is sushi vinegar -- sushi-ku. To make it yourself, mix rice vinegar, salt, and sugar in a pan, then cook it on low heat until the sugar dissolves. After it cools, mix it into your sushi rice.