It's very scary the first time you drive a car; there's so many things to concentrate on inside the car, let alone worry about what's going on outside!
Although you're sure to still be scared when you go out with an official instructor, I guarantee it'll be a lot less scary than with your partner. Driving Instructors are specially trained to deal with nervous drivers, plus they have their own brake pedal!!
Really try not to worry; when you first start driving it's normal to feel as if you're never going to be able to manage it all, but I promise you will! Before you know it, you'll be doing everything automatically without even having to think about it.
Have a nice weekend, and enjoy your first lesson! Good luck x