Since many hands have touched your crystals before you, it's a great idea to cleanse them before you work with them. There are numerous ways to do this but some of the easiest are:
1. Running your crystals over incense or sage smoke
2. Placing your crystals under running water. Be sure that your crystals are water safe before doing this one. Crystals like Selenite don't like water whereas it can really help to cleanse Quartz crystals. You can learn more here
3. Leave your crystals out in the light of the next full moon. Moonlight is said to cleanse and recharge your crystals energy. Check here
for dates of the next full moon.