Cheryl, I use paper backdrops from Calumet and they perform very well. They're usually £38 for an 11m half size roll and quite easy to take in the car and store at home.
As for the f number you should just shoot to whatever you need and the effect you want! Also f numbers mean different things with different lenses (i.e. f8 on a 16mm lens is very different to f8 on a 200mm lens).
Unless you want a defined shadow try and get your subject as far away from the backdrop as possible. If you're further away it'll throw your background out of focus more but at the cost of being further away from your subject. For some this is good, for others it's bad (so a family group that need no interaction from you might benefit from your distance whereas a child on their own would probably need the opposite).
Also getting the subject away from the backdrop makes it easier to light the backdrop without the lighting affecting your set of lights on the subject.
