Can I use a good primer such as killz brand to cover up white mold on my wooden basement ceiling? Does the area need to be cleaned or is painthng enough ?

Mold should be killed and removed before you try to seal the area. Do not use household bleach to try to kill mold. Wood is porous and the roots will not be killed, it is spent after a few minute and has no residual affect. Remember, mold and mildew are the same thing so use a good commercial mildewcide to kill the roots.