If you are overwatering your plants, or if you have a high humidity in your grow area, you may notice mold growing on the top of your soil. Mold needs organic material and moisture to grow, so you are providing the mold with a perfect environment.
The most common reason mold starts growing is if you are overwatering your plants. When growing in soil, you want to make sure the top inch of soil dries out before you water your plant again. If you water more often, you make a better place for mold to grow and you can also overwater your plant.
If you already have mold growing, take a spoon and scrape off all the mold, being careful to not let it touch any new parts of the soil if possible. Then lightly mist the whole area with a mixture of half vinegar and half water. You don't want to get the soil wet, just mist the top part of the soil with this solution. The vinegar can hurt your plant's new roots so be careful to use as little as possible. You may want to repeat the vinegar-water mistings once a day for two or three days. This should kill any mold that remained after you scraped the visible parts off.
If you've already been hit by mold once, you want to make sure that you're careful that there's not too much humidity in your grow area and that you're not overwatering your plants. You can control the mold by controlling the amount of available moisture. If you let the first inch of soil completely dry in between waterings, than that alone will probably control any more mold outbreaks.
If you continually are having problems with mold, you may want to try repotting the plant, and this time make sure that you are growing in sterilized soil. You may also want to consider a soil-less growth medium such as perlite or coco coir, which are much less likely to have problems with mold.