Selenite itself is a very pure stone hence it absorbs all negativity in surrounding easily & eject it upwards. Selenite even can Cleanse & Charge your Other Crystals and Stones if kept together with it.
Selenite is a Crystal of Higher Grounding. It is very calming stone and helps you in your spiritual working, It also assist Judgement & Insight, It forces one to be truthful and honest with themselves.
Once you receive you new Selenite, I would recommend you to do a little cleansing on crystal like all other crystal.Since Selenite can dissolve if soaked in water for cleansing , You can try rinse it under tap water for few seconds and pat it dry to clean it physically as you never know what impurities it might get with all packaging stuff, for spiritually cleansing you can keep Selenite under Moon Light over Night.
Candles,Herbs,Crystals and essential oils are solely energy work and should not be completely substituted with conventional medicines and medical methods and it is advised to consult doctor if you have serious health issues