Learn the stone system and take advantage of service gems
To get new abilities in Path of Exile, you will usually have to equip stone into sockets on tera gold your gear. Any class can use any skill, but gems do have stat and degree demands. Stone can only be equipped in slots of the exact same colour.
Skill gems: All these include an attack, charm, projectile, or trap/mine. Support gems: These gems modify supportable abilities when outfitted at a connected socket (one that has a line drawn to some other socket). The support gem must be compatible with all the stone in the connected socket in order to get an effect. As an example, a support gem that affects projectiles will not do anything to a connected melee attack stone. Make sure your linked gems are compatible!
Equipped gems also level up as you make XP, getting ten percent of the foundation XP awarded from monster kills. Your character encounters an XP penalty when revisiting lower level regions, but that punishment does not apply to gems. Thus you can certainly level gems by replaying areas that are a few levels lower than your current level.