Every monster you kill gives you a certain amount of skill point experience. No matter what level you are, 400 skill point experience will give you one skill point. monsters that are close to your level (when you select them, their name is white) give the most skill points. If you want, you can earn more skill points per monster by having a gap between your character level and highest skill mastery level(explained in detail
here), but you'll gain less EXP per monster.