If you want alot of money, I and just about everyone else would suggest two gathering professions. Gathering professions gather materials for crafting professions. For example: skinning goes with leather working because you use primarliy leather that you skinned from animals. Mining goes with blacksmithing, jewelcrafting and engineering. Herbalism goes with alchemy, which is making potions for health and mana and alot of ther kinds of potions.
Generally mining and herbalism isn't chosen together because both require you track veins, herbs on your mini map for faster gathering. The problem occurs when you want to track both at once, but can't because you can only have one form of tracking active at any time.
That said, since you already have skinning I would suggest you get either mining or herbalism. I can't reccomend one because I haven't gone very far with herbalism so I don't know the leveling dificulty compared to mining.
If you want to keep Leatherworking, then buy leathers off the AH once you start making money off skinning or go to a starting zone, like Elwynn Forest/Durotar, and kill/skin the mobs there.
You haven't screwed up your character, nor can you really. You'll be suprised how many people are actually poor in-game, especially at 40/60/70 because of mounts and all. Just remember to sell everything, and try not to buy gear off the AH because you'll probably get better gear for quest rewards anyway.
Here are some links to get you started.
Item quality- just incase you don't know what to sell to vendors and what is worth selling on the AH.
Profession powerleveling- This is a thread with all the professions and which recipes to use to powerlevel them. Most crafting professions will cost alot of money to powerlevel, but they have good guides for gathering profession powerleveling too.
Hope this helped and remember, in the end it doesn't matter what other people tell you, as long as you're having fun playing the game you're doing it right.
Good luck.
Edit: speeling.
Edited, Feb 10th 2008 10:44pm by fuzzzywuzy