static void OnWorldLoadFinished(object sender, EventArgs e)
{
    EventTracker.AddListener(EventTypeId.kSkillLevelUp, OnSkillLevelUp); //Keep this in this method as the listeners are cleared when a world is loaded.
}

static ListenerAction OnSkillLevelUp(Event @event)
{
    HasGuidEvent<SkillNames> skillLevelUpEvent = @event as HasGuidEvent<SkillNames>; //Check the Skill.SkillLeveledUp method. It sends an event when someone levels up in a skill.
    //Below I make some other conversions for you:
    Sim sim = skillLevelUpEvent.Actor as Sim; //@event.Actor is also valid for this one.
    Skill skill = skillLevelUpEvent.HasGuidObject as Skill;
    //The rest is up to your logic. Check the Skill class for things such as new level, skill name, etc.
    return ListenerAction.Keep; //ListenerAction.Keep to keep, ListenerAction.Remove to remove the listener.
}