What can I do

What to do about signing a birth certific an baby's not mine

This sounds crazy... and it is... but in some states you are stuck unless some one else is proven to be the father. Get a really good attorney!

You can ask for paternity test... get a lawyer.

Get a lawyer, then get a paternity test. The laws differ from area to area. Depending on your relationship to the mother at the time of birth and how old the child is, you may be stuck as the legal father.

Yes, if you are in the US, the laws vary by state, in a few, if you don't contest in a certain amount of time you are stuck At this point all you can do is hire an attorney get a DNA test and see if you can be taken off as the father.