God is in three Persons: Father, Son, and Holy Spirit.
They are like a united team. By nature, all 3 are God (equal in power, love, wisdom, etc...). But they are distinct Persons, who are united.
---
For example, we are by nature humans. But Father, Son, and Holy Spirit are by nature God.
When humans form a team like in sports, we say "we are one", because a team has one goal. But humans in a team are distinct from each other (one team is formed from several people - not several people glued together).
Similarly, God is One. This means Father, Son, and Holy Spirit are distinct from each other but united. And they have the same goals.
---
That's why God the Father, God the Son, and God the Holy Spirit form the Trinity (Tri + unity).
Jesus talked about all three:
"And Jesus came up and spoke to them, saying, 'All authority has been given to Me in heaven and on earth. Go therefore and make disciples of all the nations, baptizing them in the name of the Father and the Son and the Holy Spirit, teaching them to observe all that I commanded you; and lo, I am with you always, even to the end of the age.'" (Matthew 28:18-20)
Do you play sports?
Perhaps sports can help with understanding this.
Suppose a sports team is called Evergreen.
There are 3 people on this Evergreen team. Each person is an individual, but together they are a team.
When someone says Evergreen - it means that team of 3.
---
Similarly with God. When someone says God, that means the team of 3.
Individually, they are God the Father, God the Son, and God the Holy Spirit.
But united, you just say God.
God is One, because of God's incredible unity. God is more united than any sports team. Father, Son, and Holy Spirit are so close to each other and have the exact same goals. They are so close and united that you call God "Him" instead of "They".
That's why God is one. Another name for God is Trinity, which means Tri (three) + unity.
God the Father, God the Son (Jesus), and God the Holy Spirit are God in three distinct Persons. The Father, Son, and Holy Spirit are equally God, with the same abilities, powers, and good will toward mankind. God is one. We believe in one God, because God is united. We can think of being one as being close to each other and being on the same team. Similarly, God is one, because He is united in the Trinity: tri + unity.
Jesus talked about oneness in the sense of unity when He prayed for His disciples: “Holy Father, keep them in Your name, the name which You have given Me, that they may be one even as We are” (John 17:11).
----
The Father, Son, and Holy Spirit created the heavens and the earth.
In the beginning, God said, "Let Us make man in Our image, according to Our likeness; and let them rule over the fish of the sea and over the birds of the sky and over the cattle and over all the earth, and over every creeping thing that creeps on the earth" (Genesis 1:26).
Notice, God used the words "Us" and "Our".
Jesus is the Creator too.
"God, after He spoke long ago to the fathers in the prophets in many portions and in many ways, in these last days has spoken to us in His Son, whom He appointed heir of all things, through whom also He made the world." (Hebrews 1:1-2)
God the Holy Spirit was present during creation as well as the Father and Son. The Holy Spirit is often called the Spirit of God or the Spirit of Jesus in the Bible: "The earth was formless and void, and darkness was over the surface of the deep, and the Spirit of God was moving over the surface of the waters" (Genesis 1:2).
So God the Father, God the Son, and God the Holy Spirit existed since eternity past, because God is eternal and has no beginning. And God in unity created the heavens and the earth.