I'm 16 and a guy i've been forgetting things like that like if i do something or put something somewhere i forget, why?

Too many suspects: A list of possible reasons is so long, ranging from innocently benign (most likely) to frighteningly malignant (very unlikely). Adolescence is always a challenge - few of us would say it was easy if we told the truth. So much happens so fast that focus of attention (needed to remember anything) can be hard to maintain. There are mental exercises (Lumosity.com) that can build concentration. Relax.

Hard to say: Examine your lifestyle.All of us perform at our worst if we have a sleep deficit, are sick, are under stress, or have poor nutrition.At 16 you should be getting at least 9 hrs of effective sleep/day.Your diet should be balanced with some regular exercise.If re-aligning your life doesn't help, see your doc.