How many calories should i burn in the gym?

Depends: It depends on what your goals are. If you are already at ideal weight, you should burn off slightly less than the total calories consumed. If you are overweight, then burn more and vice versa. In general, every calorie you burn in the gym will allow you to take in somewhat more calories. Monitor your progress and you will find your own path. I am sure there will be many other opinions.

Calories in/out: How much weight do you want to loose. I'm assuming you are after weight loss vs. body building! You have to burn 3500 calories to loose a pound of fat... The rest is up to you. Unless you are also making major life style changes your success will be short lived.

Gym: that all depends on what your goals are, fitness or bulking up. It depends on wether you want to gain weight or lose it. There are no set amount of calories to burn. There are many tables of calories per exercise that people burn in a specific time period. It may be worth talking to an exercise physiologist, not some dude who has become a trainer after working out a lot.