There are a large number of online resources at the various levels, paid and free, to assist adoptees in their search for their birth parents. One site, the U.S. government website, Child Welfare Information Gateway, has multi-page summary of the state laws on adoption in a downloadable PDF format. Quoting from the Information Gateway publication, Access to Adoption Records:
In nearly all States, adoption records are sealed and withheld from public inspection after an adoption is finalized. Most States have instituted procedures by which parties to an adoption may obtain both non-identifying and identifying information from an adoption record while still protecting the interests of all parties.From a genealogical standpoint, adoption information from the courts was far less restrictive in the past. It has only been more recently that strict controls on the information have been implemented. Again quoting from the publication:
When an adoption is finalized, a new birth certificate for the child is customarily issued to the adoptive parents. The original birth certificate is then sealed and kept confidential by the State registrar of vital records. In the past, nearly all States required adopted persons to obtain a court order to gain access to their original birth certificates.This is an area where scams are not uncommon. If you are searching for your own birth parents, you should make sure you are dealing with a reputable business or law firm before spending money to obtain information.
For specific information on finding birth parents see the FamilySearch Research Wiki article, "United States Adoption Research."
For a timeline of Adoption History see the The Adoption History Project from the University of Oregon.