1. dehumidifier, you need the one with pump/drain tube to pump the water to a drain or outside of house.
2. get a humidity monitor for under $10, <50% humidity prevents mold growth, <35% humidity actually kills mold.
3. For existing mold, spray diluted bleach water, and brush repeatedly. For mold grown into wood, you can sand the wood down for aesthetics. Wear respirators when doing this.
4. If above methods cannot get the humidity under 50%, then you have a moisture problem in the basement.