“This, the first of his signs, Jesus did at Cana in Galilee, and manifested his glory. And his disciples believed in him.” – John 2:11
Jesus’ first miracle was private, quiet, nearly invisible. Nearly. His earliest disciples were there to see water instantly turned into fine wine. Because they did, they believed for the rest of their lives, though they certainly still had much to learn about Him.
They were there that day when Jesus revealed that He had the power of God over the elements of nature, able to change one thing (water) into something else (wine) in large quantities. They were there when Jesus quietly blessed a poorly planned wedding feast by providing better wine than what they’d had previously. They were there when Jesus transformed the water for cleansing that symbolized the old covenant of Moses into wine, that symbolizes the blood of the New Covenant in Him. They were there the first time Jesus revealed His glory just for their benefit.
The reason Jesus did miracles was to reveal to people that He was indeed the Son of God, God Himself, with all the power and glory of God. That day He revealed Himself to a handful and they believed!