These questions are pretty much all answered in the beginning few statements of the Creed.
“I believe in one God”
Christianity, building on Judaism, firmly believes that there is only one God, incorporeal, invisible, who is the maker of all things visible and invisible. He is the supreme being. That anything can be and exist is because it is given a share in His being. God is not a force, an energy. He is not a thing, but is the fullness of personhood. He is the fullness of perfect intimate communal relationship. He is love itself.
“I believe in Jesus Christ, His only begotten Son… I believe in the Holy Spirit, who proceeds from the Father and the Son.”
God, one and only, is also a self-revealing God. As God, much of His inner reality is hidden to us, for He is infinite and beyond our comprehension. However, He loves to reveal Himself to those who accepts His love. He revealed Himself as Trinity, one God, but three Persons: the Father, the Son, and the Holy Spirit. They are not three gods, but one God, and three Persons so completely joined by perfect reciprocal love they are of the same essence. God Himself though one is one perfect relationship. And it is out of this that God poured forth the same perfect love so that all things were created, so to participate with Him in the same love.
“I believe in Jesus Christ, our Lord, who by the Holy Spirit was born of the Virgin Mary, and became man.”
In the beginning, the Son, the second Person, was invisible and incorporeal, who was also the Word of God. This Word came down through the Holy Spirit, conceived by Mary the Virgin when she said yes to the archangel Gabriel’s message of inviting her to be the mother of Jesus, and became fully man, while maintaining to be fully God. Jesus became one of us in all things except sin, so that He could rightly represent us to defeat Satan, sin and death by dying on the cross, take down the gates of hell, and open wide the doors of heaven to all who believe in Him.
To read a complete explanation of the Christian beliefs summarized by the Creed, you can find that in Book 1 of the Catechism of the Catholic Church, which is available in vatican.va.