Newbie Guide:
Learning magic is an ongoing process. As you practice what you've learned, you will continue to advance and earn SP. So, if you learn Wind Strike now, you will be able to cast the Lv.2 Wind Strike. Then, once you've earned more SP, you may use it to learn Wind Strike again, and thus be able to cast the Lv.3 Wind Strike.
You can raise your level for almost every skill, so you can decide to either focus all your SP on one skill and become a specialist, or spread out your SP among several different skills.