If you're having trouble picking out your Trade Skills, the best choice to start out is Skinning. Even if you're not a Leathercrafter, Skinning is one of the easiest and fast-cash building Trade Skills in the game. More than half the monsters you fight can be skinned as well as many Critters like Sheep for Wool. With Skinning you can also harvest off monsters killed by another player, as long as they looted everything off it. Now for courtesy, don't skin a mob if the person who made the kill is also a Skinner.
Skins are used well in both Leathercrafting and Tailoring, if either of your skills are these then the more the better for you. Even if you have another one such as Engineering or Enchanting, Skinning will provide you with the cash you need, whether selling your skins to NPC's or other players. Infact, its not uncommon for a spellcaster to start with Tailoring/Skinning combination instead of Tailoring/Enchanting so you would have two lucractive trade skills. You can remove Skinning at a later time then pick another when you have made your cash flow.