Dear Siddhi,
The Alpha and the Omega are the first and last letters of the Greek alphabet. Jesus is telling us He is the first and the last, the beginning and the end. He existed before creation, and was with God in the beginning.
John 1:1 KJV
1 In the beginning was the Word, and the Word was with God, and the Word was God.
He, Jesus or “The Word”, is, therefore, eternal.
I