Professions can be trained to 4 levels of proficiency.
Apprentice
You must be level 5 for the primary professions.
This gives you 1 skill in the profession.
You may progress to 75 skill in the profession.
Cost to train is 10 for Primary professions and 1 for Secondary professions.
Journeyman
You must be level 10 to train Journeyman level in building tradeskills.
You may train this anytime you have 50 or more skill in the profession.
You may progress to 150 skill in the profession.
Cost to train is 5.
Expert
Primary Professions
You must be level 20 to train Expert level in building tradeskills.
You may train this anytime you have 125 or more skill in the profession.
You may progress to 225 skill in the profession.
Cost to train is 50.
Secondary Professions
You need to be level 20 to use the books to get expert level in most secondary tradeskill professions.
You may train this anytime you have 125 or more skill in the profession.
You may progress to 225 skill in the profession.
To train, you need to purchase a book (respective Expert book of your profession).
The book sells from vendors at 1 before discount.
Artisan
Primary Professions
You must be level 35 to train Artisan level in building tradeskills.
You may train this anytime you have 200 or more skill in the profession.
You need to find a unique special trainer which is usually found in out of the way locales (not in cities).
You may progress to 300 skill in the profession.
Cost to train is 5.
When training to Artisan in a profession that has specializations, You do NOT need to take a specialization to train Artisan.
Secondary Professions
You must be level 35.
You may train this only when you reach 225 skill in the profession.
You may progress to 300 skill in the profession.
You will need to talk to a trainer, who will give you a quest.
Completion of this quest will teach you Artisan level as a quest reward.
There is no direct cost associated with the training. Some of the skills, especially gathering and secondary skills, do not have character level requirements for some levels. All require level 5 to get apprentice. All the primary, building skills have the level requirements. All the secondary skills have a quest which is given only at or above the level requirement for Artisan.
Master
Upcoming changes in The Burning Crusade
There will be a new maximum skill for professions, at least 375.