The Skill Book, or skills dialog, is accessed by pressing (default) B. It shows all the skills your character has learned. You can drag skills from this dialog onto any slot in the Action Bar, allowing you to use those skills by pressing 1-8.
You will find that it is sometimes impossible to drop a skill onto the Action Bar. This is because some skills are only usable in a certain Stance and each stance has its own Action Bar. So, you need to activate a stance before you can either slot or use a skill which is only usable in that stance.
A useful tip is to dedicate a slot to stances so you put a skill that changes stance in, say, slot 7 then when you activate that stance, the Action Bar changes and you put the skill that changes back to the previous stance in that new blank slot 7. So you can just keep pressing 7 to toggle between stances.
When you're in a 'stance', an icon will appear above your characters health bar indicating which stance you're currently in.