I'm not a master, just a poor newbie. But the choice of lens to use is depending on how far you are from the objective, if you are close (i.e. just in front of them, then you don't need a 70-300mm lens, a 18-70 would suffice to fill the screen with the objective, in this case the model)
Now on the matter of f stop and speed, it all depends on the available light in the location, your ISO setting and whether you are using flash. If brightly lit, then you won't have much problems choosing your aperture to blur out the background or to show more...
There is no magic numbers to follow, just keep shooting and trying until you get the desired picture.