To determine the rise and run of the steps, measure the distance from the floor of the deck to the ground and divide that number by 7.5. The result will be the number of steps the staircase will need (e.g., deck is 112" from the ground; 112/7.5 = 14.93. Round up to get 15 steps).
Keeping this in consideration, how high should a step be?
The general rule (in the US) is 7-11 (a 7 inch rise and 11 inch run). More exactly, no more than 7 3/4 inches for the riser (vertical) and a minimum of 10 inches for the tread (horizontal or step). You can find some more information here as well on other stair-related dimensions.