I want to know if the baby is mine, she won't give me a dna. She says the baby is someone else and the certificate was signed by the other guy but the baby look like me how can I get her to give me a dna.

Get a lawyer and take her to court.

If she won't voluntarily submit to a paternity test, then you have to obtain a court order for one. You file for custody of the child stating you believe the child is yours. You will have to provide some proof that you had sex with her during the period in question.

The court will then order the test.