Who Should Do the Cleanup?
If the moldy area is less than about 10 sq. ft., you can probably handle the job yourself. However, if there has been a lot of water damage and the mold growth covers more than 10 sq. ft., consult the EPA's Mold Remediation page. There's information on homes, schools, and commercial buildings. The information is also applicable to other types of buildings.
If you choose to hire a contractor (or another professional service provider) to do the cleanup, make sure the contractor has experience cleaning up mold. Check their references, and ask the contractor to follow the recommendations of the EPA or the guidelines of the American Conference of Governmental Industrial Hygienists.
If you suspect that the heating, ventilation, and air conditioning (HVAC) system may be contaminated with mold, consult the EPA's Should You Have the Air Ducts in Your Home Cleaned? before taking further action. Do not run the HVAC system if you know or suspect that it is contaminated with mold — it could spread mold throughout your home.
If the water and mold damage was caused by sewage or other contaminated water, then call in a professional who has experience cleaning and fixing buildings damaged by contaminated water. If you have health concerns, consult a health professional before starting cleanup yourself.
For more information, read the EPA's Brief Guide to Mold, Moisture, and Your Home, which you can find on their website. The information is provided as a public service by the Environmental Protection Agency for educational purposes. You can get more information on mold abatement here, and get an estimate on cleaning as well.