It's level 27 now, and knows Zen Headbutt, Psybeam, Hypnosis and Moonlight. It has a Careful Nature (Crraaap). I don't have a Moon Stone yet, but I may find one with my DOWSING MCHN.
Well, you should use it when it learns Psychic, at Level 37. The other moves later that it are useless ingame: Stored Power, Telekinesis, and Dream Eater.
Moon Stones can be found on Route 6 and Giant Chasm.
Level 17. When you can give it Moonlight. Or level 47 if you want to give it Stored Power. Both of these are competitive answers.
In terms of ingame, either evolve it ASAP or at level 19 so you can use Hypnosis to help catch wild Pokemon.
So evolve yours right now.