I am 17 years old, and i usually forget the things, what i had even said, what in do?

recent or old?: If for long time then a possibility of attention deficit disorder needs to be considered. In any case see aneurologist.

Many issues: A person performs best if well rested, well fed, healthy and in a comfortable setting. Worst if opposite. If you are 17, growing or very active and not getting at least 9 hrs of sleep/d this can be sleep related. Anxiety disorders, stress, work strife, all degrade the retention of data. It would be worthwhile to check with your doc and exclude a medical process, then plan some ways to improve.