Cooking is a secondary crafting profession, which is available in addition to your two main professions. It takes various food crafting materials such as meats and fish and turn them into consumable food. The consumables give statbuffs and are used by raiders and other players seeking to maximize performance. It is a great money maker and you should have it on your main.
BigWigs is the best boss mod available currently. It is extremely customizable, allowing you to change what is displayed on an individual boss level and how boldly you want each alert customized. BigWigs is typically the most accurate and periodically updated of the available Boss Mods. It does not require any setup at all, though there are a dizzying array of options and customizations should you wish to personalize your setup.

Crafting is the art of buying materials and turning them into other materials or finished products that you can sell on the Auction house for a profit. It is one of the most consistent gold making methods in the game, but generally requires you to bind up some amount of capital in your inventory in the form of materials and finished items before they sell.

Recipes - provide in-game capability to create more kinds of items, and so are always in demand, BUT be careful; if the ingredients are obscure, and the benefits marginal, or the recipe is too common, this is not a good option. Some otherwise very good recipes drop far to often to hold value - Copper Chain Vest comes to mind. This produces an excellent entry-level item, but the recipe is available for low silver at the auction house.
Check the Auction House and if 2 [Small Brilliant Shard] is cheaper than 5 [Dream Dust], then go to Winterspring and find Qia, then buy the recipe [Formula: Enchant Chest - Major Health], and make the enchant until you run out of Shards, or until you reach 300. The recipe requires Enchanting skill 275 to learn, so you will need some Dream Dust between 265-275 to make the Shield enchant.
Potions serve more utilitarian purposes, and it's up to you to decide whether to make use of them. The most common is the Swiftness Potion, which can provide you a handy boost in speed while going after annoying quest objectives indoors. These are generally MASSIVELY overpriced, because people know how useful they are. I opted not to spend the gold since Warriors are already fairly mobile, but slower classes may benefit greatly from these.
The most simple thing is TradeSkillMaster. This AddOn shows you quite an amount of information in the items tooltip. In my opinion, DBRegionMarketAvg the best source. It tells me the price, an item was posted on all EU realms in the last 14 days. Why not DBMarket? DBMarket only shows you the items value from your server in the last 14 days. This value often gets manipulated. Some people start to post an item with a 500g value for 500.000 Gold on the auction house for several weaks. As a consequence, DBMarket is rising towards 500.000 Gold.  If you use DBMarket with 100% and post your item for 500.000 Gold, nobody will buy it.