Lost and can't gain
I am 170cm and currently 54-55kg. In the last six months I have lost about 5kg because of long term illness and now I can't get it back, no matter what I eat (and 99% of my time is actually spent lying in bed). A year or two ago, if someone told me I would one day struggle trying to gain weight I would have probably sent them to get their head checked. I think the best weight for me is about 58-62, I really don't like feeling the bones everywhere (I can't even sit on a hard surface without my sitting bones killing me!).
Anyway, before this, I have found the best way (for me) to slowly lose some weight and keep it in check. Just use the weight every morning (after going to the bathroom, but before eating/drinking) and observe if the weight is stable (in the range of about 0,5kg), rising or getting lower. You should now how much of various food you normally eat to keep your weight stable - this amount is different for everyone. Then, slightly lower the intake of food during every meal (just by 10-15%, don't go much more) and that is all. You shouldn't feel hungry (perhaps just the first two-three days) and you can eat all the food you like - this way, you will be less tempted to give up after a few days (seriously, who can stay motivated in the face of a torturous diet or exercise?). Your body gets a little less callories than it uses, so you should very slowly lose weight. When you get to your desired weight, you can eat a bit more - but don't stop weighing yourself, make sure you stay in a range of 1kg by slightly tempering the amount of food (find what is the perfect intake for you). Never do any sudden, drastic changes to your diet!
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Roughly 80% of the energy that comes from the food you eat goes into innate body functions like heart pumping blood, breathing, etc. Only 20% is use for physical activity. Even if you do work out, the maximum amount of energy you can burn is 20% - 30%. Which means that if you eat more than what your body needs, you're gonna gain weight even if you do work out. So to watch your weight, it is best to watch what you eat.
The energy that people receive from food, however, varies from person to person. A food may say that there 1000 calories in it but depending on how efficient your body is, you may not even get the full 1000 calories. And sometimes, it is not just food but a certain type of food in which your body can't efficiently digest. That's why there are people out there who eats a lot but doesn't get fat.