Have you dragged a shortcut from your book yet?
It is a hook with a worm on it.
Put a shortcut to your pole also.
Click on the pole, this will put you in fishing mode.
Click on the hook and worm to cast.
The way to catch the fish will come up if you hover over the hook and worm.
Start out on one of the docks in the canals of Stormwind. Fish there till you hit about 100 skill.