when i work out i burn almost the same calories as i eat in one day?

throughout the day i count my calories on this website, well i eat about 900 calories a day. but i go to the gym everynight afterwork. i burn about 800 calories there, sometimes even more . is that bad?

You're likely putting your body into starvation mode.  I recommend using the calculators here and using the option of "Very Active" to see how much you should be eating with that much exercise. 

 You're eating about 300-600 too few calories for a short, sedentary adult. That's without factoring in the exercise - you should not go under 1200 calories net a day if you're female, 1500 if you're male.

 And that's for adults; teenagers who're still growing need a minimum of 500 calories a day on top of that just to fuel your body.

 So yes, it's bad - you're triggering your body's starvation response mechanism. Do like Whatcolor suggests - and read this thread from 2beittybitty: Sharing Info from My Nutritionist. Follow that advice and you'll have a safe, sustainable weight management strategy.