In order to learn a skill, you must already know all of its' prerequisites at level 12 or better. You can not use a default skill as a prerequisite. Furthermore, you can not default a skill from another default skill.
To initially learn or increase your level with a skill, you must spend character points and/or time. See B81-B83. In general, 200 hours of study is worth 1 point.
Note that simply using a skill you already know is not the same as actively training in it. Every 8 hours spent on a job counts as 2 hours of training in that job. See B83 and B193.
The Game Master may optionally allow Quick Learning Under Pressure, under very limited circumstances which must relate to the immediate survival and well-being of the character. See CI114.
Most skills can be self-taught. When self-teaching, 400 hours of study is worth 1 point. Self-teaching requires access to good teaching materials. For example, to self-teach a spell requires you to be literate and have access to good textbooks. See M5.
You can effectively "study" a skill while adventuring. If you "study" skills A, B, and C, simultaneously for a period of 3 hours, you would get 1 hour per skill, not 3 hours in each. Check with the Game Master regarding how many hours could be applied to particular skills during any particular adventure. For example, Jungle Survival could count for every waking moment, but Broadsword only for the time you're actually swinging it. Note that most things you do while adventuring are just hours spent on a job. See "actively training" above and B83 sidebar.
Spending points
Unless explicitly stated otherwise by a game rule or the Game Master, spending the minimum points to learn a new skill (typically 1/2 point or 1 point for a spell) enables you to learn the skill at the level normally dictated by your attributes, regardless of the teacher's skill level. However, any advancement past that requires self-teaching or a teacher whose skill level exceeds that of the student.
Spending points does not happen instantly. If you are learning a new skill that has no prerequisites, you must spend 100 hours per point, otherwise you must spend 40 hours per point. Note that once you have spent the minimum points to learn a new skill, you are no longer learning a new skill, so the second case then applies. Check with the Game Master for the time required for special cases like buying an advantage or increasing a stat.