In levels 1-9 you just use the attack button and all you really have to do are the simple quests.
On levels 10-29 you use the skills from maplestory wikia are: Dark Sight, Nimble Body, Lucky Seven, Double Stab. The skills that are Lucky Seven and Double Stab are for attacks like Lucky Seven uses wristbands, and Double Stab uses daggers.
In levels 30-59 the skills are very helpful once they are mastered. The skills for Assassins are: Avenger, Hp Boost, Mp Boost, and Drain. For Bandits the skills are: