Before you decide that DNA testing is the way to find your birth family, be sure that you have utilized more traditional resources.
In most states, adoptees are entitled to a non-identification document, This usually comes from the agency or the state. It is information from the original interviews with the birth mother. It is called non-identification because identifying Information is usually redacted from the document, however, before deciding that there is no important information in the document, ask at the DNAadoption mailing list to have a Search Angel review the document. Often there is information in there that may be apparent to a Search Angel, who has reviewed many such documents. If you already have non-id but it is more than 3-5 years old....request it again. Times are changing and more recent non-id can have new information.
This is the best source for finding out how to get your non-id as it is referred to.
There are currently over a dozen states who have opened adoption records to some extent.