Selenite is grounding, cleansing, protective and insightful. It promotes peace and balance, stabilizes emotions and helps you to get peace of mind. The stone makes you cheerful and optimistic, and can help you to sleep better. Selenite is a self-cleansing stone that can charge and cleanse other stones.
Selenite is a self-cleansing stone, so it requires little care. It can even cleanse and charge other crystals! Still, it is a good idea to cleanse and charge selenite itself once in a while to ensure it keeps working. It's best to leave selenite under the light of the moon for a night. You don't have to put it outside, inside in the windowsill works just fine. :) Don't let selenite get in contact with water, it will dissolve. Here I explain more about cleansing and here about charging crystals. :)