First Time Grower Uses Sun System 150w Hps

Hello, this is my first time growing and i just purchased http://www.hydrowarehouse.com/images/sunlight-sun-system-hps150.jpg
My question is, How far should it be from the top of my plants!? please i must know, i had 4 plants and i put the light about less then 1 foot away from the plants and the next day they all died. I bought 2 new plants and moved the light farther away. please if anyone has any experience with this pls let me know!
...ima noob.. pls help!
 
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