You'll discover that it requires a lot of patience and time to build your skill level up to the maximum level.
You can skill up your fishing from 1 to 450 in any fishable water. This means you can stand in Orgrimmar all day long and get skillups on your fishing at the same rate as you would in a higher level zone. What decides your skillups is the number of successful catches.
1 - 300
First, visit any trainer in the main cities of old Azeroth - just ask a guard, and learn Fishing Apprentice.
Buy a Fishing Pole, it's sold by trade goods vendors and fishing suppliers. Go to the nearest water, and fish from skill 1 to 300. When you reach skill 75, you can go to Barrens and catch [Deviate Fish], the best spot is to go into Wailing Caverns and fish there, because then you won't have to move between pools to find School of Deviate Fish. You can sell the fish raw at the Auction House or if you have the recipe you can sell [Savory Deviate Delight].
Do not forget to visit your fishing trainer when you reach skill 75, 150 and 225!
300 - 450
Before you travel, gather together gear and lures that improve your effective fishing skill. At least a +5 Strong Fishing Pole and a stack of +75 Bright Baubles.
There are a lot more fishing Poles and lures.
Click Here For Fishing Poles List
Click Here For Lure List
Go to Borean Tundra in Northrend and learn Master rank from Fishy Ser'ji (Horde) in Warsong Hold.
Even with good gear and lures, initially you will catch a lot of junk fishing in Northrend, in Borean Tundra you will need 475 effective skill to fish without catching junk. If you are higher level, around 80, you should go to Wintergasp instead of Borean Tundra, you can make the most gold by fishing there, at the start, around half of your cathes will be junk, but the amount of junk will gradually decline.
Do not forget to visit your fishing trainer when you reach Fishing skill 375!