Profession Levelling Guide
There are 2 sorts of professions types, primary professions and secondary professions.
The Professions
The primary professions include the 3 gathering professions: mining, skinning and herbalism.
The primary professions:
Alchemy
Blacksmithing
Enchanting
Engineering
Herbalism
Leatherworking
Mining
Skinning
Tailoring
The secondary professions:
Cooking
First Aid
Fishing
You can only practice 2 primary professions. You can, and should, skill all secondary professions. Any profession can be “unlearned” at any time. If you choose at a later time that you again want to skill the unlearned profession, you will have to start from square 1.
Skill caps
When you first take up a profession, you will have a skill cap of 75. This means that you can skill it up to a maximum of 75 before you need to go to a trainer to raise it further. There are differences between the primary and secondary professions.
The only difference between primary and secondary professions is the level you need to be able to reach Artisan (highest level – cap 300). For Primary professions you need skill 200 and you must have reached Level 35. For secondary professions you need skill 225, have reached level 40 and there is a quest to raise the skill cap to 300.
Here are the skill levels for primary professions
Skill level – Profession Level
0 – Novice (cap at 75)
50 – Journeyman (cap at 150)
125 – Expert (cap at 225)
200 and Level 35 – Artisan (Cap 300)
How to skill Professions efficiently
For those of you who just have started playing and do not have an alternate to back you up with bags, I would suggest to get Herbalism as your first profession, as second Skinning. The reason is simple, with a new character you are lacking bag space and herbs stack up to 20. Since you need a tracker active to find herbs more easily, skinning is the only other feasible gathering profession. TIP: Make a priority of buying more bags! The Auction House is a good source of cheap bags.
Until level 8 or 9 it is not too important to know which professions you want to practice, latest by level 10 you should know. As you start to leave the first noob area, you will start to encounter that you require a minimum skill to be able to skin / mine / gather.
Excluding the gathering professions (which bring you gold) wait with levelling these until you reach level 40 and have bought your mount. There are 2 simple reasons for this. To level a profession (other than the gathering ones) costs a lot of gold. The second reason is, you make more coin at level 40 than you do at level 10. 
If you are below level 35 and already have more than 100 gold, you can start to skill that profession.
To be able to quickly skill your profession, send material from the starting areas (silverleaf / copper / leather – and other materials) to a secondary character so that you have more bag / bank space on your main character.
How do you know how much material you will need? Check www.thottbot.com and plan a rough plan. Most patterns / recipes can be used up to 15 times until they turn yellow, skill up only on orange items for a guaranteed skill point, at yellow you may have to produce 2 or 3 to get that skill point.
To conclude my levelling guide, here a short description of each profession:
The gathering professions
Below you find a short description of each. To note is that only one type of tracker can be active at any point in time. This makes a combination of Mining and Herbalism incompatible. One could take both and still find ores and herbs, but you will miss a lot of your potential.
Mining
Profession type: Gathering / Tracker
Required Items: Mining Pick
To be able to find Ore, you should have the tracker active as it will show you any ore veins in your immediate area. Mining also includes smelting, this refines your ore into bars. With mining you will, once in a while, find gems. To be able to mine, you are required to have a mining pick in your inventory which uses up 1 bag space.
Metal bars sell well on the AH.
Ores can stack up to 10, Bars up to 20.
Herbalism
Profession type: Gathering / Tracker
To find Herbs, you should have the herb tracker active, this will show you all herbs in your immediate area. You need no other tools, which makes this a great first profession if you don’t know what to do.
Herbs are a big seller on the AH.
Herbs stack up to 20
Skinning
Profession type: Gathering
You can skin almost any creature in WoW except humanoids. You get either Leather or hide from a creature. To be able to skin a creature you need to have a skinning knife in your inventory.
Hides stack up to 5, Leather up to 10.
Alchemy
Profession type: Crafting
Matching Profession: Herbalism
Alchemy is a good profession, it has many “buffs” that raise stats for a limited time. Drawback is when you die, the buff is gone and another potion has to be taken. Some may think “I can make my own healing potions.” If you really need healing potions, kill any humanoid, they drop them.
Blacksmithing
Profession type: Crafting
Matching Profession: Mining
With blacksmithing you can make your own mail and plate armour as well as some very good weapons. The higher level items sell very well, however, requires a big investment in time and gold to create the better weapons. Store a lot of material and all gems you find!
Engineering
Profession type: Crafting
Matching Profession: Mining
I have not tried engineering yet. What I saw is that there are many interesting items that an engineer can make.
Leatherworking
Profession type: Crafting
Matching Profession: Skinning
I was surprised to see that a leatherworker can make mail armour. You will need tons of leather, therefore I would suggest that you store some material of the lower levels. There is no way around you having to go back to lower levels to get that required leather. It will be faster as you can quickly kill the creatures required, but you will have to go back to the easier areas.
Tailoring
Profession type: Crafting
Matching Profession: none / Enchanting
With tailoring you will need a lot of cloth. To save you a lot of bag space, ask a friendly tailor to convert your cloth into bolts. While cloth stacks up to 20, bolts stack only to 10. Only with linen does this make no difference as you need 2 linen cloth to make 1 bolt of linen. Wool saves you some space, silk a lot.
Grind a lot on humanoids, In fact, grind ONLY on humanoids and you will still not have enough cloth.
If you combine this with enchanting, don’t store too much linen and wool. A lot of this stuff drops in the deadmines which you will anyway spend some time to level enchanting.
Enchanting
Profession type: Crafting
Matching Profession: none / Tailoring
Enchanting is the most expensive profession to grind! You have to disenchant green (or better) items to get dusts, essences or splitters. This is expensive because instead of selling these green (or blue) items, you destroy them. If you choose enchanting, start only at level 40, then go to the Deadmines and run through a few times.
Don’t expect to reach 300 for a good time!