Overall this is a strong response. However, I would advise that you don't use phrases that make it sound like you have already been accepted, which I'm assuming you haven't. Confidence is good, but you don't want to sound arrogant.
When I attend USC, I plan to major in Biological Sciences or Human Biology at the College of Letters, Arts and Sciences.
Maybe say "If I am admitted to USC, I plan to major in..."
I plan to pursue a degree in Biological Sciences to later attend USC's Keck School of Medicine. I decided to major in Biological Sciences because it has three specialties, of those three I plan to choose an emphasis in molecular and computational biology.
As it is now, this part of your essay is a little awkwardly worded. I think you should start by saying that you hope to eventually attend USC's Keck School of Medicine, and then go on to say that this is one of the reasons that you would like to major in Biological Sciences. Then you could say that another reason is that it has three specialities, of which you would choose molecular and computational biology.
By taking Human Biology I will be able to focus more on the human organism, which will be of great help since I will become a pediatrician.
Again with sounding a bit full of yourself. I think you should say something like "Taking Human Biology will allow me to focus on the human organism, which will help me achieve my goal of becoming a pediatrician.