Daisy, I suggest that you take all the science you can and take algebra and college algebra. If your school has anatomy/physiology be sure to take that.
As far as being able to handle ER and surgery. You don't have to do ER and the rotations are not very long in fact some nursing schools do not even have a rotation in the ER. surgery is part of the OR clinical and at my school we are only doing a 2 day OR clinical.
You said you want to be an OB/GYN nurse I am not trying to discourage you but many OB/GYN nurses may become part of a team if the women needs a C-Section and that is defintely considered surgery.
As for clinicals it depends on what nursing school you are at. You will do community clinicals in some programs, Labor and delivery, Med-surg, psych, pediatric, ICU, OR, Orthopedics those are some of them. For one particular are you may do a clinical anywhere from days to 6 weeks some may be longer. It all depends. You are in your junior year now is the perfect time to go to your couselor and talk to him/her about colleges you are interested in with nursing programs
and calling/writing/visiting the colleges to talk to people their including nursing advisors.