To get new skills in Path of Exile, you'll usually need to equip stone into poe currency sockets in your gear. Any course can use any ability, but gems do have stat and degree requirements. Gems can only be equipped in slots of the same colour.
Skill gems: All these add an attack, charm, projectile, or trap/mine. Support gems: These gems modify supportable skills when outfitted in a connected socket (one with a line drawn to another socket). The service gem must be harmonious with all the stone in the linked socket so as to get an effect. For instance, a service gem which affects projectiles won't do anything to some connected melee attack gem. Make sure your linked jewels are compatible!
Equipped gems also level up as you earn XP, getting ten percent of poe market the base XP granted from monster kills. Your personality encounters an XP penalty when revisiting lower level areas, but that punishment does not apply to stone. So you can certainly level gems by replaying areas that are a couple of levels lower than your existing level.