Mary was greatly troubled at his words and wondered what kind of greeting this might be. But the angel said to her, 'Do not be afraid, Mary, you have found favor with God. You will be with child and give birth to a son, and you are to give him the name Jesus. He will be great and will be called the Son of the Most High. The Lord God will give him the throne of his father David, and he will reign over the house of Jacob forever; his kingdom will never end.'
'How will this be,' Mary asked the angel, 'since I am a virgin?'
The angel answered, 'The Holy Spirit will come upon you, and the power of the Most High will overshadow you. So the holy one to be born will be called the Son of God. Even Elizabeth your relative is going to have a child in her old age, and she who was said to be barren is in her sixth month. 'For nothing is impossible with God.'
'I am the Lord’s servant,' Mary answered. 'May it be to me as you have said.' Then the angel left her." (These verses were copied from bible.cc)
I know that was a lot today, so we will break this up. In the beginning of these verses, Gabriel tells the Virgin Mary she will be the mother of Jesus. This is the same angel who spoke to Zachariah about John the Baptist.
Gabriel tells the Virgin Mary she will be the mother of Jesus. Before Gabriel came to her she had absolutely no idea, to my knowledge, she would give birth to God's only begotten Son.
This would be a miracle because, well, because birth is a miracle in itself. Besides that, she was a virgin. How can a virgin give birth? Mary had faith. She believed. The angel Gabriel even said, "for with God nothing shall be impossible." (Luke 1:37)
Once he said that, Mary basically said, let what you say come true.