First, you want to make sure the mold isn't cause from an active water leak. One way to do this is to purchase a moisture meter (home depot sells them around $30). Next, take a sheet of paper and draw a grid on it that represents the wall you're going to test. You're going to want to test the wall every three feet. Start at the top and work your way down making a note on your grid of the moisture levels. This will give you a good idea if there is a leak behind the wall. You'll see a pattern in the moisture levels. This way, you'll have an idea if you need to open up any walls or if the mold if simply caused by moisture in the air. If you believe the mold is being caused by a leak, you will need to call a professional remediation company unless you feel prepared to do it yourself.
If the mold is caused by excessive moisture there are a few ways to treat it. Run your air conditioner, this will help to dry out the air. Check your filters and keep them up to date. You may want to treat the unit with a Mold Bomb Fogger from BioCide Labs. If you call them, they will walk you through the whole process.
Mold is everywhere indoors as well as outdoors. The problem is when the conditions are right and they start to feed and colonize. Treat this with extreme caution because mold has the potential to make you very, very sick. Using bleach or natural products to treat mold only contributes to the problem. Most species of mold is spores are chlorine resistant and the bleach will turn it translucent but does not kill it. The mold will always return. You need to treat your environment with a biocide that has anti-funal, anti-microbial elements. The company that sells the mold bombs make a kit that will treat your entire home. Give Biocide labs a call and they will help you get the correct product and walk you through all the steps. This is a much cheaper alternative to hiring a remediation company.
BUT...if you feel like you're in over your head, go ahead and call in the pros. Like I said before, mold is serious and needs to be remediated asap. Do not take a chance with your health.