I have a hard time understanding when people are joking or being serious and i have a hard time understanding social rules i'm 19 what could cause these problems?

You may have a mild form of autism where it is difficult for your brain to interpret social clues. This would best be diagnosed by a PhD Psychologist who has a special interest in autistic spectrum disorders. While there is no "medicine" for this, being aware of what is causing the symptoms you have noticed can be very helpful in determining other behavioral therapies.