For the typical ground setting with firm earth you will need one foot of shock. This however will vary slightly with the different soil formations. The best system to use would be. 25 feet for every foot above ground. This formula is very standard and is used by many in the North American pool industry.
The amount of shock needed for a 15' swimming pool depends on a number of things such as the ph level of your water, how dirty the water is and how many gallons of water the pool holds. A 15' pool holds about 4500 gallons of water and therefore would need about 200 grams of shock.
For that small of a pool I would add about 1/2 ounce of shock and then check your bromine level. Add accordingly until it's up where it should be. That is between 3 and 5 as recommended by the health department.
Use a one time use bag of pool shock every 3 weeks. I recomend that you purchase a test kit to determine the quality in the pool. These are very inexpensive and very handy. If your Ph is below normal range. Add 1/2 the bag of shock. If your Ph is very low add the entire bag of shock.