Alchemy- Most people would think that flasks is the way to go as an Alchemist, however, the ONLY time this is true is when you are an elixir spec. Sometimes, this will not even help. A great way to make some starter gold in Alchemy is through the use of your epic gem transmute. This is especially true if you are a transmute spec(which I highly recommend). The mats for an epic gem transmute are fairly cheap especially since you can trade frozen orbs for the eternals. Alchemy is best used when grouped with another big money profession such as Jewelcrafting. However, I would not rely on it being your main source of income.
Blacksmithing- A safe market to get into with Blacksmithing is the Eternal Belt Buckle market. These will always be in demand. Every time a new belt is won, this belt buckle is the first thing bought. Mats are fairly cheap and you will always be able to make a profit off of them. Blacksmithing has more areas available to it to make gold, but this is a market that will always be there.
Enchanting- The entire enchanting market in general is a low risk market. The constant need for enchants will always have people running to the auction house over paying to get them five minutes before they raid. There are many different enchants you can put on scrolls to make profit, however, the one I want to focus on for starter cash is "Fiery". It is an easy farmable drop from Blackrock Depths. It should take you no longer than an hour to farm. This enchant is used all the time on BoA gear. Put this enchant on scrolls and watch out for people trying to undercut you, and you should be able to make decent profit off of this.
Engineering- So pretty much engineering is one of the hardest professions to make gold with in WOTLK. There is just not many ways to earn you gold. In Cata they will be changing this. However, your best bet until then will always be pets. Make them and pray that nobody else wants to make 200 of them too.