I am a retired police detective, but you don't need to be a detective. You don't need to pay anyone. Follow these steps and you will come full circle to find your own truth.
1. Ask your Adoptive parents. Get all the info you can. Paperwork, dates, everything.
2. Contact the adoption agency. There has to be a keeper of the records even if it no longer exists. Your mother may have left consent to contact you. They usually will contact her on your behalf.
3. Get your original birth certificate. Some states don't allow this. Go to the states vital record website and get the paperwork. Some counties allow you to look through birth records. You can figure out later which one is you.
4. Create an ancestry.com account. Everyone should have one regardless. Ancestry has millions of members now. Put your birth name as you, if known. If you have to put "unknown" in the mother and father slots do that. Add 18 years to your own birth date for your parents birth dates. If you have your mothers name put in what you know. Let Ancestry do the work for you. Log in the next day and you make have a new leaf. You don't need to pay for an account. Use a desktop computer and roll over the previews. You can get plenty of info free. Make your adoptive family tree for practice. Save your two week trial for DNA if you don't have a mother name or birth name. Remember "find a grave" results, the 1940 census and some other stuff is free. New leafs will pop up when you add new stuff. Make your spouse's tree or your kids. Basically to know what you are doing and how to use the hints and judge the accuracy of the search results.
5. Send for an ancestry DNA kit. You can later use your raw data results on gedmatch.com and find out if your parents were related ect. Once you get your dna you will have it forever... look at it as an investment. Look for coupons and sales on retailmenot and other sites. Sometimes it is free shipping, sometimes it is a discount. If you don't have names this is what you need to do. For less than 100 dollars you will find at least 100 4th cousins or closer. Be nice to them and they will help you. Look at their trees. Build your own off of theirs. Have a nice profile pic, your own private family tree. People will be more willing to help you if you are an active user. Wait until you get your DNA results if you have limited funds. Use your two week FREE trial then.
6. Go to legacy.com and search all of obituaries. Put your mothers maiden name in the keyword slot. If she was mentioned in her grandmother or mother's obit. ect. It will show up.
7. Facebook. You can create a page for your search and target advertise to people from her highschool year ect. You can also make a picture of your info and share it everywhere. Once you get a name look up facebook accounts. Only you can make the ID. You will know if people look like you or not. Ask friends who know you well. It can be hard to see the forest for the trees.
8. If you know the town, search in historical newspaper archives. Peekyou.com radaris and other people search sites might be helpful to if you have only a maiden name to give you birthdates and possible married names. Don't pay. A savvy searcher can get this info from collecting it from multiple sites.
9. Adoption registries. You can also look up how to "google dork". It will turn up info quicker. Put yourself on a registry just in case. You will have six weeks waiting for your ancestry DNA results to do all of this other stuff. Soundex and adoption.com are the big ones.
10. Remember non ID info can be fictitious. Adoption agencies had their own agendas to keep people from finding one another. Some info you believed was true about yourself may not be. It is also good to know your genetic make up from DNA results. If it is different than what you were told by the adoption agency, trust NONE of it. Most birth mothers don't know about leaving letters in the file and if the agency gives you no help it doesn't mean they are dead, never looked for you or don't want to be found.
Please remember to view my most popular post, 13-reasons-why-adopted-children-are-not lucky. If you are new to the adoptee community you need to realize you aren't alone. You may face people who tell you to be grateful and leave well enough alone. Most adopted people have positive reunions. Those who do not are at least grateful not to live in ignorance, no matter how blissful.
If you have additional search tips please leave them in the comment section to help other adopted people. If you are waiting to be found, don't wait. Chances are no one is looking for you. Doesn't mean they don't want to be found. The happiest relationships are amongst reunited siblings.