Humans reproduce by sexual intercourse between a man and a woman
Matthew 1:18 - 25
ESV - 18 Now the birth of Jesus Christ took place in this way. When his mother Mary had been betrothed to Joseph, before they came together she was found to be with child from the Holy Spirit. 19 And her husband Joseph, being a just man and unwilling to put her to shame, resolved to divorce her quietly.
For follow-up discussion and general commentary on the topic. Comments are sorted chronologically.
Mary conceived through the Holy Spirit, God does not do anything without the Holy Spirit. Every word God speaks is through the Holy Spirit, thats why when Jesus left he said that I dont leave you alone but with the Helper who will teach you all things.
So you believe the Holy Spirit had sexual intercourse with Mary and she conceived a child?
This is indeed a difficult concept to get our heads around, that a woman could conceive without having intercourse. But the Christian walk is all about exercising faith, believing and trusting a God who does things that are beyond our own understanding.
To address your last comment, no, the Holy Spirit did not have intercourse with Mary. If this were true, Jesus would not have been born by a virgin, and He could not have been our perfect sacrifice for our sins.
Then how did it happen, you might ask? We don't know. There is no logical explanation. This can be aggravating to a person who tends to think logically, one who wants to see how things work in order to believe and understand them.
All I can say is that you simply believe, which is what Jesus tells us to do. In John 20, Jesus addresses this issue after His resurrection when He spoke to Thomas:
V. 29 Jesus said to him, "Have you believed because you have seen me? Blessed are those who have not seen and yet have believed."
Hope this helps.