Join a guild that has a tailor and you can buy bags cheap or get them free. After a couple chars, I realize that making money is more important at lower levels than a profession.
I went with herbalism and skinning. It's amazing how much some of the stacks of lower lvl herbs go for in the AH. Oh and BTW, everything you gather should be sold in the AH. I always check the going rates and undercut by a bit. My stuff always sells within 24hours.
As a third gathering profession, take up fishing. It takes a bit to get going, but if you like the battlegrounds, you can fish while you wait for battle and get it up to 100 in no time. Stacks of Oily Blackmouths and Firefins and Stonescale Eels are worth 2-8 gold a stack, maybe higher in some realms.
When you get to 40, you can level whatever profession you want pretty fast to 250. In the meantime, you will have enough gold to buy your upgrades. Choose a profession that interests you. I like tailoring for the bags mostly. They sell pretty well once you get to runecloth bags. I also made tuxedos for anyone who wanted one in my guild.