The only thing you have to bear in mind is that it's easy for you to level your character faster than your gathering professions, and find yourself adventuring in an area where many of the collectibles are too difficult for you to collect. If that happens, you'll find that because you aren't able to gather as much, your professional skills aren't increasing as fast as your XP, so that in the next zone you enter it'll be even harder to gather stuff. It's a runaway train, and the only way to stop it is to nip it in the bud: if you find yourself in an area full of, say, mageroyal, or tin ore, and your gathering skills aren't yet good enough to collect it, then the best thing you can do is take a break from adventuring, and go back to the zone you just left and start gathering peacebloom or copper ore until you increase your professional skills to the point where you can start using them in the new zone. If you don't do this, your professional skills will stagnate through lack of opportunities to improve them.
Skinning has one slight advantage over mining and herbalism: you rarely need to go out of your way, for two reasons: because the animal you are skinning is one that you will often have killed yourself, and whose warm corpse you will be wanting to loot, anyway; and because very often when you are killing things, other adventurers will be killing things right beside you, and you can skin their kills without much extra work!
So, you can happily begin your professional life by picking two of the three gathering skills and sell what you collect in the Auction House. If you do this while levelling, you will be rich, because the fruits of your labour are always wanted by other professions.
Another popular strategy is to pick one gathering profession and a manufacturing profession that uses the raw materials you gather. If you're a mage, herbalism and alchemy are popular choices, hunters often choose mining and engineering (so they can make their own ammunition). Miners often choose jewelcrafting or blacksmithing, and herbalists who don't take alchemy often choose inscription. Don't bother with inscription, unless you are prepared to make a real industry out of it. It's the sort of industry that you would want to devote all your bag space to, and then some. It's the sort of profession that you want to devote a whole alt to.
Other popular combinations are skinning and leatherworking, mining and blacksmithing. Don't pick these. The idea is that you can make your own armour and save yourself a ton of money. Trust me, it never happens this way. You simply end up wasting your gathered materials making items to skill up your manufacturing profession, and you end up having to dump these manufactured items on the AH for less than the cost of the raw materials, because nobody wants them, not even you: you will get armour that's roughly as good as you can make, just by questing and looting.
What is important is that you realize that you are not stuck with these professions forever. As you reach higher levels, you will easily be able to afford to throw one away and quickly level another from scratch with materials you buy in the Auction House. Let me briefly tell you what each profession involves:
- Alchemy: turning plants into potions
- Blacksmithing: Turning metal into armour, weapons and so on (you must be at an anvil to do this)
- Enchanting: disenchanting armour, weapons and so on into magical essences, and using these essences to enchant other pieces of armour, weapons and so on to increase their stats.
- Engineering: making ammunition, and fun gadgets (bombs, rockets, goggles, toys) that mostly are only usable by engineers
- Herbalism: plucking plants (for resale or reuse in another profession)
- Inscription: making ink from herbs to create glyphs (and offhand items and trinkets).
- Jewelcrafting: prospecting raw gems from metal ore, making jewelry (necklaces, rings, etc) and cut gems that increase their user's stats.
- Leatherworking: Turning leather into armour.
- Mining: digging up metal ore (for resale or reuse in another profession)
- Skinning: turning dead animals into leather (for resale or reuse in another profession)
- Tailoring: turning cloth into clothes (including cloth armour).
Of course, I didn't know all this when I chose my professions: I chose tailoring and enchanting - a common combination, but probably one of the worst for levelling mages.
First, I chose tailoring because I wear cloth armour and I thought it was a great way of using all the cloth I was picking up off kills; and it is, but the items you can manufacture as a low-level tailor are not the sort of items most people want to buy. The exception to this is bags, but the standard bag that everyone buys is the Netherweave Bag, and it's a long time before you learn to make those.
Second, I chose enchanting, as I thought it a magely thing to choose; and it is, and it also has a sort of gathering skill built in, disenchanting. This is handy because it allows you to disenchant armour that would otherwise be useless to you into magic essences (shards, dusts and so on) that you will use for enchantment, or for resale. Those enchanting materials can net quite a pretty profit, but in general, enchantment is a drain on your wealth rather than a benefit, as the low-level scrolls of enchantment you make while levelling are generally not as valuable as the mats you used.
Once you have maxed out a manufacturing profession, then the story changes; then you will be able to make rare and in-demand items that level-capped players need. For instance tailors can make spellthreads, which every top-level spellcaster needs and will pay a fortune for. Each profession has something like that, and will start to generate income for you, as well as providing special items that only you can use. Once you reach the level cap, and have enough money to be able to power-level a profession (i.e. buy all the materials you need in the auction house to get from level 1 to level 450 in the profession in a few hours) then it's time to review your professions and perhaps choose a profession for the bonuses it can provide you. Until then, choose at least one gathering profession, and either a second gathering profession, or a manufacturing profession that uses the fruits of your gathering or provides you with items you know you will need. Don't rely on being able to sell what you manufacture for more than the cost of the raw materials.
Finally, you should always pick up the secondary professions, cooking, fishing and first-aid. They don't take up one of your two professions, so you can have two primary and all three secondary professions (or four if you are a rogue: they get lockpicking as well).