The main five proof verses of the virgin birth are found in the angel’s appearance to Joseph in Matthew. 1:18, 19, 25 and Gabriel’s appearance to Mary in Luke 1:34, 35
Matthew 1 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. 20 But as he considered these things, behold, an angel of the Lord appeared to him in a dream, saying, “Joseph, son of David, do not fear to take Mary as your wife, for that which is conceived in her is from the Holy Spirit. 21 She will bear a son, and you shall call his name Jesus, for he will save his people from their sins.” 22 All this took place to fulfil what the Lord had spoken by the prophet: 23 “Behold, the virgin shall conceive and bear a son, and they shall call his name Immanuel” (which means, God with us). 24 When Joseph woke from sleep, he did as the angel of the Lord commanded him: he took his wife, 25 but knew her not until she had given birth to a son. And he called his name Jesus.
Luke 1: 26 In the sixth month the angel Gabriel was sent from God to a city of Galilee named Nazareth, 27 to a virgin betrothed to a man whose name was Joseph, of the house of David. And the virgin’s name was Mary. 28 And he came to her and said, “Greetings, O favoured one, the Lord is with you!” 29 But she was greatly troubled at the saying, and tried to discern what sort of greeting this might be. 30 And the angel said to her, “Do not be afraid, Mary, for you have found favour with God. 31 And behold, you will conceive in your womb and bear a son, and you shall call his name Jesus. 32 He will be great and will be called the Son of the Most High. And the Lord God will give to him the throne of his father David, 33 and he will reign over the house of Jacob forever, and of his kingdom there will be no end.” 34 And Mary said to the angel, “How will this be, since I am a virgin?” 35 And the angel answered her, “The Holy Spirit will come upon you, and the power of the Most High will overshadow you; therefore the child to be born will be called holy—the Son of God. 36 And behold, your relative Elizabeth in her old age has also conceived a son, and this is the sixth month with her who was called barren. 37 For nothing will be impossible with God.” 38 And Mary said, “Behold, I am the servant of the Lord; let it be to me according to your word.” And the angel departed from her. 39 In those days Mary arose and went with haste into the hill country, to a town in Judah, 40 and she entered the house of Zechariah and greeted Elizabeth. 41 And when Elizabeth heard the greeting of Mary, the baby leaped in her womb. And Elizabeth was filled with the Holy Spirit, 42 and she exclaimed with a loud cry, “Blessed are you among women, and blessed is the fruit of your womb! 43 And why is this granted to me that the mother of my Lord should come to me? 44 For behold, when the sound of your greeting came to my ears, the baby in my womb leaped for joy. 45 And blessed is she who believed that there would be a fulfilment of what was spoken to her from the Lord.”
Additional evidence comes from the genealogy of Jesus in Luke 3 following Heli (Mary’s father) back to Nathan, brother of Solomon (Joseph’s ancestor), on to Adam.
Now Jesus himself was about thirty years old when he began his ministry. He was the son, so it was thought, of Joseph, [the son] of Heli, (Luke 3:23 NIV)
The second “[the son]” before of Heli verse 23 is actually not there in the Greek text, since Jesus was “of Heli” his maternal grandfather, but Joseph was not Heli’s son.
Finally, note Galatians 4:4, where Paul says that God “sent forth his Son, born of a woman, born under the law”