At most, you will have to eat 200kcal more than someone with similar stats. You just aren't eating enough. Don't make excuses like 'high metabolism', spread out your meals and eat calorically dense foods.
hey 50-53kg is a bit too skinny for someone who's 5"6 imo! (health wise). I'm 5"3 1/2 and 50/51kg and im average/ slim ish.
But anyway eat healthier. Once you've cut most of the junk out your diet you can reduce your portions slowly over time if its still too much. When it comes to exercise, start with normal exercise such as jogging and biking then, if you want, move to HIT training once your body has adapted. Diet is more important that exercise though.