Christians believe that God created the Universe. He chose to create it out of love for humanity. Some may argue that he created it because he wanted to be worshipped, therefore it was selfish his creation however Christians would debate this. They would argue that as God is Omnipotent he doesn’t need to be loved.
One of the main theme running through Christianity is the theme of Love. God is full of Love as we should all be.
For Christians it is not important how God created the Universe, as they believe that is beyond their own imagination and for them it doesn’t really matter. This is their faith. There are different arguments for the compatibility of God and Science. Read about them here. There are two versions of the Creation story in the book of Genesis.
Genesis 1 is also known as the 7 Day creation story. This is the story that God created the Universe in 6 days and rested on the 7th.
Genesis 2 is the story of Adam and Eve. Where God created humans first and placed them in a Garden. This provides an account of how sin entered the world
There are different types of Creationist Christians (You can read about this in more depth here) In general they believe that the story in the Bible is the literal truth.
Liberal Christians take these as stories or parables. Stories that help tell a more complex idea. This is how many Christians would then link in these ideas with Scientific Theories like the Big Bang Theory and Evolution.
This links with the Philosophical ideas of Telelogical and Cosmological ideas.
See this table for different interpretations of the Genesis stories.