Are you sure about that with crafting, Kalizaar? As I understand it, crafting xp (alternatively known as "Inspiration" to separate it from standard xp) is completely level-independent. The only level that matters when you are looking at your crafting skills is your overall level in that particular craft; for example, I dig up some iron ore (no xp for that), then refine it into iron ingots (very small amount of Blacksmithing inspiration, which will put me at level 1 Blacksmithing). I then create an iron dagger (given enough mats, I could create a dagger at any level up to 15 even though my character is, at this point, only level 1). The creation gives me some more inspiration, and I gain a little bit more by breaking it down again to try and retrieve some of the mats that went into making it.
After several creation/destruction cycles, my Blacksmithing levels up to 2. I can then spend a point in, say, Metalworking to allow me to forge Steel weapons; I could also, at BS level 2, spend a point in Keen Eye to make ore deposits within 20m of me glow. The higher skills within the Blacksmithing tree become available as I level up the crafting skill itself, culminating in being able to put a point into Temper Expertise when my Blacksmithing reaches level 10. All of which, theoretically, I could still do while my character is level 1 (given the skill points from somewhere other than leveling).
Going back to Hygeliac's original question, you will gain xp in your skill lines as you explore, fight and complete quests - I _think_ (not 100% sure) that you gain a small boost in any skill tree that you have opened up, you definitely gain a boost in any skill (and associated tree) which is on your action bar, and a bigger boost according to how often you have used that skill or a skill from that tree.
Short version: weapon and armor skill lines will open up by using an item from that tree, you can then add skill points to expand your 'mastery' of it and be able to add new skills to your bar which will enable you to level it up faster. Other skill trees require an initial investment of a skill point into one of the associated skills, after which they work in much the same way. Crafting xp is earned from creation and destruction of items associated with that craft, and leveling it is independent of your other skills.