Using active skills are limited by the level of your active job:
Debuffs and Self-Buffs: To use these skills, the level of learning the skill minus your current job level must be 10 or lower.
Attacks and Team-Buffs: To use these skills, the level of learning the skill minus your current job level must be 20 or lower. This also counts for the level of your teammates receiving the buff.
Additionally, when you have raised your job level just to the limit, the skill will only have 50% of the effect. Each 1 level you gain closer to the skill's learning level, you regain 5% of the effect, and capped at 100% after 10 levels. For example, when your current job's level is 20 level lower than a team-buff skill's learning level, you can use the skill but it only lasts 50% of its duration; if you gain a level, it becomes 55%, and so on; after you gain 10 levels, (this means you are now 10 levels lower), you can use the skill with full duration again.
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The recommended skills depend heavily on your active job as lot of skills scale with different attributes. It also heavily depends on what you're trying to achieve. Generally, these are useful sub skills:
Medical Treatment: For additional healing and regeneration, can severely cut down time. Can be used to heal other people as well. Also provides mana regeneration at higher levels. Useful for most jobs, but more useful for people with high Wisdom as it increases the healing from First Aid.
Pet Sounds: Used to heal and buff pets. Very useful if you use a pet as you would otherwise have to use items to restore its health and mana.
Close Combat: Provides mana restoration and an evasion buff, as well as a defense buff and health restoration at higher levels. Useful to reduce downtime on casters.
Sneak Attacks: Provides the ability to increase movement speed and a debuff that shuts down an enemy, as well as the ability to steal from monsters. Useful on solo-ers.
Nature Magic: Provides mana regeneration and a single target freeze, as well as spell reflection and an area freeze at higher levels. Mostly useful in PVP, but can be used for the mana regeneration as well.
Spellcasting: Skills unlock a little later than the other recommended skills, but provides great mana regeneration at the cost of health, an area effect attack, elemental defenses, and an area slow. Very useful for casters.
Different Dances: Provides two area effect attacks very early on, as well as a targetable health and mana regeneration at higher levels. Useful for new players as it allows them to dispatch large quantities of low level monsters very quickly on any job.
Commoner Skills: Not combat effective, but essential for item collection and alchemy. Very common since almost all non-Commoner passive skills have no use until level 30.
The usefulness of most other sub skills are heavily dependent on the active job. You could use Seasonal Magic very efficiently on a Wizard, but it would be awful on a Blademaster.
Upon each new level reached, 3 skills points are rewarded. It is also possible to increase your skill points for any job by using springs. Every 10 levels, players are given given a Life Quest that awards a non-tradable spring upon completion. These springs provide an additional 5 skill points when used.
It is also possible to purchase tradable springs from the item mall, however they only provide a single skill point as opposed to the 5 from the Life Quest obtained springs, and there is a limit to how many can be consumed, which increases by 1 every 2 levels. Commoner's Springs are the only springs that can be obtained via other means than completing a Life Quest.
Skills can be leveled up when a player has reached a certain level, and has a certian amount of skill points. On the first page of your skills list, these are the first few skills you will be using. When you get a new job, you'll see one or two skills (depending on your job, i.e. Martial Artist). As you level up, you will learn a new skill or more with your skill points, and level up previous ones. On the second page of your skills list, you'll see either more different skills, or better versions of them. They can only be obtained by leveling up a certain skill to level 5. Note that some skills require level 3, 4, or 6 of a certain skill (Such as Mega Medicine, Mock Monsters, Arrow Formation, Snake Formation, Crane Formation, etc.).