For simplicity's sake, start with 1.5 cups of water for one cup of dry long grain white (Basmati or Jasmine) rice. If you need to make a larger quantity, reduce the water ratio a bit closer to equal parts. Sushi rice is a whole different thing, which I may do another instructable for later to go with my sushi sushi sushi
Remember you more than double your rice volume, so don't cook a full cup of rice per person! So, two cups of rice and two and a half cups of water will yield just over four cups of cooked rice. I find that is just right for four people, and we usually have leftovers for quicky fried rice the next day.
First put the rice and water in the pot. Place the pot on a level surface and put your (clean) finger in to touch the bottom of the pan. Make a mental note of where the water level hits your finger.