How do you choose the shutter speed? It's always going to be a balance between slow enough for the background to be blurred and fast enough for the subject to be sharp avoiding general camera shake and tracking error. It also depends on how far away your subject is: the further away it is, the slower it will appear to move. That said, a bigger lens will close that distance: view the moon through a 1,000mm lens and you can see the moon actually move through the viewfinder.
You'll need a lens that can focus fairly quickly and the camera should be set to continuous autofocus. Then, track the object with your camera in a smooth motion ideally keeping the object stationary in the viewfinder and take your picture. Rather than just one frame, I usually fire off a short burst of frames as not only does it give you subtly different backgrounds, but it also improves your chances of getting one in focus. And that's something you really need to know about panning: a slow shutter speed and a moving object (and a moving camera too) means that many of your pictures will not be adequately sharp seeing them head straight to the waste bin, but that's just the way it is with panning.
When it does work, and many times it will, the result is bound to put a smile on your face, and has for me provided some of my favourite motor sport images. When you're starting out with panning, do experiment, especially with the shutter speed, to see what works. Check your work as you go and change the shutter speed accordingly. And as always, practice makes perfect,