Surfing Waves

I've read the page about duck diving on the site but im stil having problems. When i go to duckdive i get the board down but i miss the back of the board with my foot and the tail gets caught by the wave and i get thrown off.

So you can sink the nose fine? Once the nose is down as far as it can go , use one of your legs to kick down (push down on the tail hard with your foot) the tail and dive down+forward (where the nose is)

yep, once youve got the nose sinking lift your body up with your arse in the air, this will help you find the tail with your foot - use the raised bit on the back of your deck pad to find it and you can lift your other leg into the air if you need to get more weight on the tail.
At the last moment you need to push the tail right down with your foot while thrusting yourself forward and under the wave (i also kick my feet once im under to keep forward momentum) and then pull on the board in a scooping motion once your under the wave. This should bring you to the point where your chest is back on your board underwater and the board is angled slightly nose up so you come up nose first on the other side to prevent being dragged back and loosing your grip on the board.
pratice in the lineup while your waiting for waves until you can sink the whole board and dive so youre whole body is underwater without a wave to duck and youll find it easier when you have the power of the wave sucking you in.
A good duckdive under a big steep wave just as it throws out a lip is a great feeling. a bad duckdive in that situation can lead to being pulled back over the falls - which is not such a great feeling.

Practise on a sunny day and keep your eyes open - you can (just about) see how deep and how long you need to be underwater, and you can 'scoop' the board back up at the right moment

Use your knee instead of your foot. Then progress to your foot if you need that much more drive.. but in anything up to head high, you should be fine driving up with your knee on the tail.