How to do crafting in this game ?

I'm almost at level 12 and still don't know how to do crafting ,can someone explain please at what level can start and how do I do that ?
For example blacksmith , armorer, leather worker ?
Thanks

You can start at level 10. Just go to the guild hall for the craft you want to learn and sign up, there will be a quest above the receptionists head.

Blacksmit and Armourer is in Limsa Lomina, Leatherworker is in Gridania.

To actually 'craft' you'll need to first complete one of the quest Bixillarla mentioned and then equip the tool you are given in your main hand slot. Crafting is affected by two main stats: 'Craftsmanship' which is an indication of how much progress is gained when you synthesise an item and 'Control' which indicates how much progress is gained when you add quality to an item.

Generally speaking, all quality actions require crafting points or CP. Maxing out the quality bar guarantees the item you craft will be high quality assuming the item actually has a HQ version, but getting it there may take some effort. Every quality or progress action will also reduce the durability of the item by 10 points. Different items have different durability levels. Ingredients used in other crafts often have 40 durability, while finished complete items may have up to 80. If durability reaches zero, the synthesis fails and there's a fair chance you may lose all the ingredients.

All crafting requires the use of crystals, shards or clusters which can be mined from the gatherer classes Miner or Botanist. These are lost whether you complete or fail the synthesis, even in instances where you might recover failed ingredients. You'll get through a LOT of these, so be sure to keep plenty on hand.

To begin synthesis, you need to open your crafting log (Keyboard 'N' by default) and select a recipe from the list. As long as you have the necessary ingredients, you can hit synthesize to begin. Substituting high quality ingredients will increase the quality that the craft will start with, so it can be very beneficial to use HQ items if your intention is to make a HQ final product. This is crucial later on for the more difficult crafts, such as Silver Brocade or cashmere.

Crafts share a lot of identical skills with each other, but all of them have their own unique skills which you can use with any other craft. For example, Blacksmith gets Ingenuity, Culinarian gets Hasty Touch (the only quality gain which costs zero CP) and Weaver gets Careful Synthesis. These can absolutely make or break your synthesizing in the later levels so you'd be well advised to level a range of crafts.