Hopefully, you already know why you want to be a doctor. If you do not, do not apply this year. You won't get in. If you are unsure, seek out relevant medical related activities and find ones that you enjoy and ponder about why you enjoyed them. For example, did you enjoy working in the ED because you like to be under pressure? Or did you like research because you like finding out answers on your own?
If you know why you want to be a doctor, and have several relevant ECs, but just need help articulating that into a PS, might I suggest working on your ECs first? That helped me to avoid repeating things and to make my application cohesive.