Herbalism, typically. Skins sell decently. But it's only used for Leatherworking, and rarely in Tailoring/Blacksmithing. But people always need herbs. Inscription and Alchemy (which is a popular profession) require herbs. There's a bit of a hump within the profession (if you pick it up at level range when you start) at the beginning, but after that, it's the easiest gathering profession. Plus you get XP for mining ore and picking herbs. So if you got the both of them; more XP!
But yeah, you can have 2 main professions and all secondary professions. Which are..
- First Aid
There are also daily quests for Fishing and Cooking (which can be done at any
level) for more bonus XP.