Am 21 years old .. i have a problem on my memory i can't remember good .. what medication should i have ?!

Depends: You need to get evaluated for the reason of your memory issues. It may or may not require medications.

See below: I would recommend discussing your symptoms face to face with you primary doctor, getting a work up from him and asking for a referral for an assessment of memory done either by a neurologist or a neuropsychologist. Get a complete evaluation, physical and neuropsychological and then discuss your options. Good luck!

Hard to speculate: People preform at their worst in any way when sick, when they don't get enough restorative sleep, when their nutrition is not balanced & in unfamiliar settings. You need to start with what you can fix, and a checkup with your doc would be good. If nothing is found to explain your problem, additional testing with an educational psychologist would be reasonable. There is no "memory pill"