You ask what you should do with the baby you had
outside of marriage and before you were a Christian, if you now want to marry another man.
As you probably realize, when Jesus forgives our sins, He does not always take away the consequesces of our sin. This child who has come into the world, even if it was through your sin, is still a person who is created in God's image. (He/she is a person who will be able to talk to and
listen to God.) He/she is your responsibility and needs to be loved and cared for by you. You need to think very carefully before considering marriage now. You would have to ask the man if he is willing to adopt him/her as his own. If he is, he will be showing a very real love for you, because it will not be easy. On the other hand, if he rejects him/her, you should question how much he loves you, because your child is part of you.
If, however, for some reason, it is really not possible, he/she could be given out for adoption, but then you would have to give him/her up. Foster care is another possibility, but that is not always a happy solution. You need to think about what effect your actions will have on your child, and do what is best for him/her.