Hi, my name is ____. I'm from ____. I attend (school) as a (year), majoring in ____. My hobbies are: ____. Aside from school, I participate in (organizations and work). And most importantly, I want to be a pharmacist!
20 seconds?, better not use um,er,uh, or the like. You'll run out of time.
I'd say start with first realization pharm was for you and what you've done to prepare yourself since. That's if you've only got 20s.
Other options would be scholastic history including accomplishments/awards, background as to why pharmacy if you have compelling historical story (my ??? got sick, decided pharmacy somehow based on that.).
Problem is with 20 seconds you can only pick one and it better be the shortest one.