Ok so this is what happened my daughters mother didn't tell me she was born but I found out a day or so later and come to find out she let her new boyfriend sign the birth certificate and gave her his last name without me knowing and I was wondering if that's illegal and what I can do to fix the situation?

It may not be illegal unless there was no possibility this man could have fathered the child.

So what you need to do is file in your local Family Court for joint custody of the child. The court will order DNA testing. Once you have been proved to be the father the court will rule on custody and visitation. I would strongly suggest getting an attorney.

I would also file with your state's Putative Father's Registry.