How do I get a Profession?
You have to find an NPC that can teach you their Profession. Ask other people where you can find certain trainers, or discover them on your own.

Using Professions
After you have trained or purchased a Profession, hit "P" to bring up your abilities book. Then, look for a new icon that has been placed in there for that skill. You can drag this icon over to your action bar to make it easier to find and use. If you can't fit it on the first row of your action bar, place it on a different row by left-clicking on the scroll arrows to the right of your action bar (it's ok if the icon is still selected on your cursor).

Profession Limit
You can learn all of the secondary skills, but can only choose two professions.

Character Level Limits
In order to prevent low level characters from mastering high level skills, a certain character level limit is required for each level of profession. You will need a minimum character level to learn a new level of professions. Apprentice requires level 5, Journeyman level 10, Expert level 20 and Artisan requires level 35.

Skill Increases Gray - You cannot increase your skill by making these items or harvesting these items.
Green - These items give you a fairly poor chance to increase your skill. You can still do it, but its going to take a lot of gathering/creation to up your skill with these items.
Yellow - These items give you a reduced chance of increasing your skill. It's still pretty reasonable to try to increase your skills with these items.
Orange - These items give you your best chance of increasing your skill. For many of the Professions, making an orange item will always increase your skill.
Red - Your skill is not high enough.
Secondary Skill Training
Expert and above skill tiers for secondary skills require you to find the books to teach you them. Artisan level requires a quest to be completed.

Other Information

• Some Professions allow you to use the trade screen to interact with other people's items. For example, enchanters can enchant items directly in the trade screen. Lockpicking and some other Professions can also be used via the trade screen.
• If you attempt to learn a recipe you already know, you will now see a message that says you already know it.
• Some recipes are intended to increase your skills in that Profession rather than to create an item to sell to another player.
• Some Epic player crafted items can be created that require raiding.
• Generally, recipes with white names are store-bought and ones with green or blue (or purple) names are drops from monsters. So if you see some white recipes for sale, a person just bought the item from a vendor and is trying to re-sell it for a profit.
• When gathering Herbs or Mining, if you see gray dots on your mini-map those are items found below you or in a nearby cave.

• Buy stacks of Profession components from other players. That is faster than collecting them on your own! Have your friends save up components too.
• Devote a certain bag to carrying your Profession items. You can even pick a different-colored bag if you have one, to make it easier to find.
• Know which classes will need the products that you can create with your Professions: Mail : Warriors, Paladins (Hunters and Shaman can start wearing mail at level 40)
  Leather : Rogues, Hunters, Druids, Shamans
  Cloth : Mages, Priests, Warlocks
  
• Know which Profession components to save while you are fighting. You can sell them to other players or give them to friends, rather than selling them to the merchants: Blacksmithing : jewels, metal bars, linen, wool, leather...
  Engineering : jewels, cloth, leather, mechanical items, metal bars...
  Leatherworking : leather, ruined pelts, hides...
  Tailoring : linen, cloth, wool, silk...
  Cooking : meat, various monster parts...