Many skills are available to anyone who wants to learn them. Examples of these would be common tasks like riding a horse, fighting with a staff, and basic survival skills. Teachers are available in all cities and many villages and other areas who will gladly teach these skills to anyone who wants to learn them (and has a little coin to spare). They may not be the best teachers in the world, but then again, some of them might.
Most skills are taught only to members of a certain clan. This does not mean only one clan teaches the skill, just that you must be a member of one of the clans that does in order to learn it. For example, specialized fighting techniques are not readily available to the general public, but any military clan would surely teach them to its members, as well as any other clan whose members see a fair share of combat.
Over the course of a characters lifetime (which is unlimited in length) a character may join any number of clans, and thus can have the opportunity to learn almost all the skills in the game (some are race specific). If the character leaves a clan, she retains her knowledge in all skills she has learned, and can still practice them, though she may not be able to return to that teacher for the accelerated learning curve.