The county that you were born in will have the birth certificate with your adoptive information. The county where your adoptive parents were living at the time of your adoption will have your birth certificate with the information about your birth parents. If you know where your adoptive parents were living at the time of your adoption, you can then write a letter of petition to the court (Circuit Court for that county) to open your records. If granted, you will be able to go into that courthouse and they will show you all of your file including the original certificate of birth. There are actually several circumstances under which your petition could be granted. I have done a petition for myself and one for a brother in Rock County. Both were granted.
There is also a state run program called the Adoption Search Program. This has fees though which vary depending on your income. If you can afford it, they give you a non identifying information package and then try to locate a birth parent. I would recommend the petition letter first.
You can also write to your state representative or senator for help and a well written letter to the editor of the Wisconsin State Journal might be helpful too. Just try the letter of petition to the court first and then go from there.
Btw, adoptions only legally change parent and child relations. All others are not effected. Siblings are still your legal siblings because they did not get a choice in relinquishing the relation.
Good luck and chose the path that is best for you when you get your information. I chose contact and some was bad but others were very good. Even the things that turned out bad, I still don't regret.