How can I have a dna test even if i signed the birth certificate and believes that I am not the real father?

I presume that the child is still very young.  The fact that you signed the birth certificate does not prevent you from getting a DNA test.   If the mother will not agree to a DNA test, you need to file a Motion with the Family Court.  If you are confident that you do this properly without a lawyer, you should do this as soon as possible.  If you are not confident and you can pay the money for good legal representation, you should retain a good Child Support Law Firm as soon as possible.    Our charges vary, based on how much time I anticipate that we will need to spend.  Generally speaking, the legal fees and costs tend to be no less than $2,500.  To accommodate clients who cannot pay the fee up front and ask for "payment plans," we accept payment by credit card (VISA, MasterCard or American Express), which allows the client to make manageable monthly payments on his/her credit card. It is very important that you be completely comfortable with the law firm you retain, before retaining that law firm.  It is also very important that the law firm be completely comfortable with representing you.  Therefore, I always make myself available for an initial brief telephone call with each prospective client, to get more facts.  There is no charge for that phone call.  If you think that you would like to retain our law firm, please call us at 973-455-1220 to set up a mutually convenient date and time for the initial brief telephone call.  Please also take a look at our Firm Website (www.salvaggiolaw.net).  It contains a lot of helpful information. I wish you the best.   David F. Salvaggio, Esq., SALVAGGIO LAW GROUP LLC