Use the directions on the bag of rice that you have purchased, however add 2 tablespoons each of mirin and seasoned rice vinegar to the water before cooking. If you can't find directions, do the following:
2 cups rice
2 Tbsp. seasoned rice vinegar
2 Tbsp. mirin
2 cups water
Bring the water to a boil and add the vinegar and mirin. Now add the rice and bring to a boil once again. Allow to boil for about 1 minute uncovered, reduce the heat to a simmer and cover. Simmer for 20 minutes and then remove from heat. Let stand for 10 additional minutes and allow to cool before preparing sushi. This quantity makes about 4 cups of rice which should be enough to make sushi for about 4 people. Double the recipe if you need to make more for guests or healthy appetites. Any leftover rice can be used for fried rice the following day.
Some prefer to add the rice vinegar to the rice after cooking, applying about 1 teaspoon per cup of cooked rice. I personally have found that using the vinegar in the cooking water disperses the flavor more evenly. Feel free to experiment both ways to see which method you like better.
Some individuals will swear that soaking the rice for several hours prior to cooking improves the flavor. I have tried this and could not tell much difference. White rice is rather bland anyway and to me it is the taste of the fish that is most appealing.