Hey there! I see you're already a Shepherd now, so you're on the right track Everything about ranks and the point system is explained here. Just allow me to quote the part that specifically addresses your question:
"How do I rank up?
By completing Quests, or earning Merits you'll see your score grow. One hint for you, the harder Quests are worth more points then Quests that are easy. "
Hope this answers your question! Feel free to ask if anything is unclear, or browse through the help section.
