To join a trade profession, find the appropriate Master in one of the major cities, and speak with him or her. The NPC will prompt you to join the school. Make sure this is the profession you wish to join - once you join, you are a member of that profession forever, and cannot leave.
Each craft has at least one associated craft which you can gain skill in, at a percentage of your primary craft's skill. Weaponcraft - 75% Armorcraft, Siegecraft; 40% Tailoring, Fletching. Armorcraft - 75% Weaponcraft, Tailoring. 40% Siegecraft, Fletching. Tailoring - 75% Fletching; 40% Armorcraft, Weaponcraft, Siegecraft. Fletching - 75% Weaponcraft, Siegecraft; 40% Armorcraft, Tailoring. Spellcrafting - 40% Alchemy; 1% Siegecraft. Alchemy- 40% Spellcrafting; 1% Siegecraft.
To craft an item, drag your craft icon to the Quick Bar, and press it once to open the crafting menu. Right-click on each item to show the recipe. Drag the item to the Quick Bar. Once you've purchased or obtained the appropriate recipe ingredients, and the necessary tools, press the item's hotkey and you will begin crafting the item. Some items will require you to be near a specific crafting station, such as a Forge or an Alchemy Table. A green bar will appear, and begin moving to the right. When the bar fills completely, a check will be made, and you will either create the item, or fail.
On orange and higher items, a failure will have a chance of destroying some of the ingredients. When the item is made successfully, it will be placed in your inventory. You will receive a message and an audible queue when the item completes, and a text message will appear, informing you of the item's quality. You will also be informed if you receive a skill increase from the completion. Gray items will not give you any skill increases, but will craft significantly faster, since the item is so farbeneath your skill level. You have a random chance to create an item of 94 to 99 quality, and a very small chance of creating a masterpiece (100 quality) item on gray items only. Over a skill of 1000, you have a 2% chance of creating a masterpiece item, regardless of it's color.