Too little calorie intake - effects?

I keep reading that not getting enough calories can cause your body to go in to starvation mode. Every time I write down what I eat for the day, I am under calories. If I didn't write things down, I'd be over, I'm sure! Also, I understand a little about the body storing fat in reserves and in certain places on our bodies and using protein and or fat for fuel, but I am still confused on a few things. If at the end of the day, I am short on calories, do I force myself to eat if I am not hungry, and if so, what am I supposed to eat? I will be going to bed and in the past if I eat right before I go to bed, I wake up starving with stomach pangs. Currently I am eating 3 healthy meals and healthy snacks in between and not eating after 7pm and usually in bed by 9pm. I just want to get rid of a lot of lower belly fat and regular excess overall on the rest of the body. I am moderately active at work, so my body is used to moving around alot and I know I need to have muscle development to burn more fat, but what can I do to understand and make it easer? I