I think the previous post was quite ignorant. Wanting to look for your biological parents does NOT mean an adoptee is unhappy or doesn't love their adoptive parents. It is a normal, natural need to know where we come from. Look at American's obsession with geneology - very few people aren't interested in where their family came from or what their heritage is.
mia1981, I would suggest starting with the agency that placed you for adoption. You can often get non-indentifying information about your biological parents, such as their ages at your birth, their physical descriptions, interests, hobbies, jobs, etc. Depending on what state you were adopted in, you may have right to access your original birth certificate or information.
There is also a registry called ISRR. If you submit your information, and your biological parents submit their information, you will both be notified of a match. It is not a searching service, but what's called a "passive registry." To my knowledge it's the largest and most well-known registry.
ISRR
Be careful about posting personal information on random websites to try and make a match. Always keep some information to yourself so you can verify a potential match if someone contacts you claiming to be a biological relative.
Finally, I suggest that you try to connect with other adoptees and read information about the search and reunion process. There may be support groups in your area or online groups for people who are searching. I'm not from MI so I'm not aware of the resources specifically available there, but I'm sure there are plenty. I found "Birthright: the guide to search and reunion for adoptees, birthparents, and adoptive parents" to be very helpful even if it is 14 years old. Some of the resources are out of date, but the message is timeless.
Searching requires a lot of patience and emotion, and there are no guarantees. I wish you all the best in your quest to find more about yourself.