I hate to sound like a kill joy, but seeing as you said you know "basically nothing" about horses I would really recommend that you stay away from this. It would be different if this was a dead-broke old school master, but this is a pregnant mare. It really takes a lot more experience to properly care for horses than it looks. You can read all the books you want but when it comes down to actually doing it, it's a LOT different (trust me on this one).
There are other, better ways for you to enjoy horses before you rush into buying one. I know the offer of a FREE horse is SUPER enticing- and a foal to match! But the money on the pregnant mare, possibly training the mare later, caring for the foal and training the foal will far exceed your expectations- the horse world is VERY expensive, don't even start about what's needed for a mare and her foal.
Now that I've said that:
Are there any barns around you? If there are, I would really recommend volunteering there, even if you can't afford lessons for some reason. It will provide you with invaluable experience that you'll be thankful for after getting your mare.
Read, read, READ when you are not at the aforementioned barn.
As for punishing a horse, usually waving your arms or brandishing a lead rope (not hitting them, waving it at them) usually works well enough. If a horse does not heed that, a light tap or smack on the body follows. If THAT does not work, make your horse MOVE. Take her out of the cross-ties/etc and circle her if need be. She'll quickly learn misbehaving is no fun.