fishing

First you must learn fishing as a secondary profession. You can find a list of fishing trainers here:

http://www.wowhead.com/?search=fishing+trainer

Once you have learned fishing and you have a fishing pole you must equip the fishing pole into your primary hand. Then you stand next to any body of water and click your fishing button from the General section of your spellbook.

You can get to the general section of your spellbook by pressing "P" then selecting the foot icon or top icon labeled "General" in your spell book. You will then see an icon for fishing somewhere in that section that looks like a fishing pole. Drag that to your action bar and click it each time you want to cast.

You can also open your spell book by clicking the button that says "Spells and Abilities" next to your action bars if you are using the default user interface.