Look at how I am holding my camera here:
Even without the long lens, that is how you should normally hold your camera: left hand under the lens, and turned so that the four fingers are below the lens, and the thumb left/above.
Why? Because this way, you stabilise the camera more. And unless you are using a tripod, stability is everything.
To zoom, use thumb and ring finger of your left hand – hand stays where it is. And to take a portrait shot, with your right hand, move your camera a quarter turn up – the left hand stays as it is.