My partner who I have been with for just over 6 years wants us to get married but before we do he wants to adopt my 11 years old daughter ,we also have a 5 years old son together ,so this would make all our surnames the same,and something he has wanted to do for a long time now, my daughter doesn't know who her father is and I haven't seen him since she was born,he isn't on the birth certificate. So that wouldn't be a proberm either, My question is who and where do we go to do this. As I haven't a clue where to start

You start with a lawyer.

Most states require you to be married for a specific amount of time before a stepparent adoption can take place, but some states allow it to happen at the same time as the wedding--not many though.

You also will need the biological father's permission for this to happen.

A lawyer will walk you through the steps you need to take for a stepparent adoption in your area.

Your partner is putting the cart before the horse. Most states require you be married BEFORE they allow an adoption.

The father not being on the birth certificate WOULD be a problem, since the courts will REQUIRE that he be identified and that he relinquish his rights before approving the adoption. At the very least you will need to show a good faith effort to contact hem. That's also where an attorney comes in. The attorney will advise what steps you need to take to get the bio father's rights terminated.

By the way, there are dozens, maybe hundreds of threads in this forum asking the same question. Its always a good idea to browse around a bit first before asking your question, Often you will find the answers that way.