The first point to understand about becoming a foster parent in the United States is that this is an issue managed by the individual states, not the federal government. There are some clearinghouses for information online regarding foster parenting, which touch on both the legal aspects and the practical challenges. A few such sites are those of national organizations created to support foster parenting and foster families, such as the National Foster Parent Association and the Families Like Ours organization. The genuinely national organizations that deal with this issue will usually have chapters in all 50 states, each with a local contact and staff who will be more than happy to discuss the process of becoming a foster parent with you. If you have a particular interest in fostering a child with special needs, you might want to visit the National Dissemination Center for Children with Disabilities Web site; they have a section devoted entirely to foster parents
However, in addition to consulting with some of the private support organizations (there's no reason to limit your outreach to just one of these types of groups), you'll have to consult with your state's specific government agency that is responsible for licensing or approving you as a foster parent. The national organizations have the lists of requirements by state and can give you the contact information of the right office. Also, a simple Internet search with your state's name and "foster parent" or "child welfare" should do the trick to get you to the right state agency. If that's not working for you, go to your state's main government page and do the search on that site.
If you're interested in more general information about foster parenting in the United States, you can check out the federal government's Child Welfare Information Gateway site, which publishes a report on foster care statistics (http://www.childwelfare.gov).