To fish you¡¯ll need a fishing rod. These are sold for fishing supply vendors who also sell various baits and lures which can be used to temporarily increase your fishing skill. You¡¯ll want to get the fishing rod and then some bait that is around your level whenever you pick up your fishing skill (they are right next to fishing trainers). The fishing rod is actually a weapon and as a quick interesting fact it does use your fishing skill to calculate the to-hit percentage. However, you¡¯ll likely find the damage is not worth using it as an actual weapon.
Once you¡¯ve got yourself a rod and some bait (fishing with bait helps a lot) you¡¯ll need to find a pond or lake or really any body of water. You¡¯ll also want to move the fishing skill from your spellbook to your action bar. This is the actual ability that will allow you to fish.
So, do you have a body of water, a fishing rod, and your fishing skill (it looks like a hook) on your action bar You¡¯re ready to fish! Now, what I do is zoom my camera in to first person perspective (you don¡¯t have to) then use the fishing skill. Assuming you¡¯re close enough to water and your skill is high enough for the area (you can use bait/lures to increase it if you¡¯re not high enough) then you¡¯ll throw out your line and a bobber will appear on the surface of the water.
Now, the mini-game starts. You watch the bobber until you see a fish bite. As soon as it does you ¡°reel in¡± by right clicking the bobber. Too slow and the fish will get away. Too soon (before a bite) and you¡¯ll get nothing. If you succeed a loot window will open giving you the option to loot the fish. If you fail, well just repeat the process all over again.
You¡¯ll gain skill points depending on the level of the body of water you¡¯re fishing in vs. your current skill. It¡¯s generally a mystery unless you want to put a good bit of research in it. However, it¡¯s my opinion that you should fish in one area until you¡¯re not getting good skill point gains anymore. Then move on a higher level zone. Remember, zones determine what level a body of water is ¨C not the body of water itself. For instance, Elwynn Forest will always be fishable while something in Outland will require a good bit of fishing skill to fish at.
Hopefully with this information you can head out start your career as a fisherman of Azeroth. The only other thing you need to know about is what exactly you can do with fish. Well, you can eat them raw. You can cook them (which makes Fishing and Cooking two very good skills together). You can throw them away, if they are too low for you to use.