1. Learn to fish (and cook). If you're going to be fishing for the mount, you might as well learn to fish and cook while you do it. The mount itself drops from fishing pools in Northrend, so if all you're doing is going for those, it'll take you a little longer to level than just sitting in Stormwind and leveling fishing (which you can definitely do), but you might as well up your profession while you do it. El's Angling is the guide of choice -- follow it closely and you'll be 450 Fishing/Cooking in no time.
2. Find Fish. If you follow El's guide, you'll figure this out along the line, but even if you've already leveled fishing, you'll definitely want to make sure you've got the Find Fish ability, obtained from the Weather-Beaten Journal, which you get from -- you guessed it -- fishing. This will help you find fish pools on the minimap tracker, and since you need to fish in pools in Northrend to get the mount, it'll be a huge help, if not invaluable.