Quests are a very small thing in MQO. It will take a certain amount of quest items to complete, and they will have a relatively small reward. You can get gold, gems, and relics from a quest, but it will be in small quantities. The quest has no cost whatsoever so it is just a little extra bonus.
Quest items are by default found at a 20% rate per attempt, but this can be increased with the quest buff. However this is currently a very weak part of the game so it is recommended to not use many(if any) gems on this.
You can automatically start a quest 75% of your level on the quest page, or you can look in town for custom Tradeskill quests from lvl 10-250. Using these shorter town quests can be useful to get the quests completed titles.
You gain 1 gem per 100 quest level bracket. A lvl 199 quest would give 1 gem, a lvl 201 quest would give 2 gems.