If you complete the mentor quest as a mentor with apprentices, you get Meridian crystals (not mandatory for every run).
You need these crystals to learn the meridian skills, which are all passive. You also need qi, which you get from qi pills. Qi pills are relatively easy to earn, because you get them as a reward in the daily quest "Death Wave". But you can also get Qi pills here and there in some quests and events.
However, some skills need Beihai crystals (red skills). Beihai crystals can be dropped during treasure hunts and can be obtained from time to time during events or in the event shop.
To be able to access the Ren tree, you first have to have the skill "Templermacht" in the Du tree fully skilled. Templar power requires Beihai crystals.
Everyone should upgrade at least the first four Meridian skills, as they increase resistances, among other things.