There are many, many frum woman doctors out there.
Like SJS said, prepare early, and decide if you think it is the right career for you, and then go full steam and give it nothing but your all.
(Not to hijack the thread, but) Being a doctor, especially an OBGYN for the frum community (as a distant relative of mine is), is an extremely rewarding career, and serves Klal Yisroel in a way that very few women are able to do.
Hatzlacha