trying to loose weight

I'm 16 I'm either 5'3 or 5'4 they always say both when I go to the doctor/hospital it's always a different height, and I weigh 137 as of right now. I'm using this app and it says for me to eat 1,606 a day? But I always end up eating less calories because I'm barely hungry throughout the day. Is that good or bad to eat less than what I'm supposed to? And as for the working out I started 2 days ago and I could only workout for like 5 mins it was painful I did wakeup sore the next day any tips to workout longer & not be as painful?