To level up quickly in Blade & Soul, prioritize the main story quests as they offer the most experience. Supplement this with Daily Challenges (accessible via F8) and Faction Quests. Don't neglect your Hongmoon Training Room (F12) for practicing rotations and earning daily rewards. Utilize Experience Charms and other buffs whenever possible. Joining a clan can also provide experience bonuses. Focus on completing quests in zones that are appropriate for your level to maximize experience gain and minimize time spent on weaker mobs. Always pick up and complete side quests in areas you are already traversing for the main story.
