You'll only get a C-section if there's a medical reason for it. I was planning on a vaginal birth, had a completely normal pregnancy with no issues, went into labor naturally the day after my due date, and 20 hours later (long story short) ended up with an emergency C-section because DS's cord was wrapped around his neck, inhibiting his ability to breathe when I started pushing (VERY RARE, please don't think it's likely this will happen to you, not trying to scare you, just give an example). There are lots of reasons to need a c-section, but you wouldn't know it this early on unless you (or baby) have a medical condition that would necessitate one. If you've had a c-section before you can opt for one the next time, I guess that's the only other way you would know this early.
I ended up having a c section because my hips never widened so my LO got stuck in my pelvis. I seriously wished that I had a vaginal birth the c section hurt so bad and I really couldnt get off the couch for the first 2 weeks. But theres really nothing that you can do to make it where you only have a vaginal birth.
Usually they ask you about family medical history and then determine whether you're eligible for a planned c-sect but if you're healthy and ask for a vaginal birth, they'll have to honor it unless there would be any ill effects on baby or yourself.