The question: Why is the turning of water into wine considered as the first miracle .. though Jesus had performed miracles before.
If Jesus performed miracles (at least one miracle) before turning water into wine it would be logical to conclude that the miracle in Cana of Galilee was not the first.
The task is to search the scripture to ascertain if the turning of water into wine is indeed the first miracle or not.
I assume that we all agree that Jesus did not perform any miracle before he was identified by his cousin John the Baptist. Jesus was unknown in terms of his messiahship before the day of his appearance at the baptism of John.
“John answered, saying unto them all, I indeed baptize you with water; but one mightier than I cometh, the latchet of whose shoes I am not worthy to unloose: he shall baptize you with the Holy Ghost and with fire:” (Luk 3:16, KJV)
None of the enemies of Jesus (the Pharisees and the Sadducees) claimed that Jesus did any miracle before his baptism at the Jordan River.
After his baptism by John, Jesus spent 40 days in the wilderness.
“And there came a voice from heaven, saying, Thou art my beloved Son, in whom I am well pleased. And immediately the Spirit driveth him into the wilderness. And he was there in the wilderness forty days, tempted of Satan; and was with the wild beasts; and the angels ministered unto him.” (Mrk 1:11-13, KJV)
The keyword is "...immediately"! Not a day after...it was immediately. Hence, he did not call anyone to join his ministry.
Therefore, the statement of John the Baptist and his conversation with the Pharisees in Jn.1:19-28 took place about a day before or the day of the end of Jesus's abode in the wilderness. Jesus went to the Jordan River after his wilderness experience in Jn.1:29: it was a day after John conversed with the Pharisees.
Day 2
“The next day John seeth Jesus coming unto him, and saith, Behold the Lamb of God, which taketh away the sin of the world.” (Jhn 1:29, KJV)
Where was Jesus coming from? Coming from the wilderness?
Hence, we can see that Jesus couldn't have performed any miracles during these times.
Jn.1:29 says "The next day". This was the day after John's conversation with the Pharisees in verses 19-28.
“And John bare record, saying, I saw the Spirit descending from heaven like a dove, and it abode upon him.” (Jhn 1:32, KJV)
'The next day', John the Baptist reported what happened sometime in the past. His disciples heard John's statement.
Day 3
“Again the next day after John stood, and two of his disciples;” (Jhn 1:35, KJV)
This is another 'next day'...this was the third day that John had the conversation of verses 19-28.
On the day of verse 35, two disciples of John left him and followed Jesus. One of the two was Andrew (Jn.1:40) who told Peter. Verse 35 possibly took place at least a day or two after Jesus left the wilderness. Hence, no miracle was performed by Jesus.
Day 4
“The day following Jesus would go forth into Galilee, and findeth Philip, and saith unto him, Follow me.” (Jhn 1:43, KJV)
'The day following' would be the day after verse 35! Again, no miracle was performed.
Day 5
“And the third day there was a marriage in Cana of Galilee; and the mother of Jesus was there:” (Jhn 2:1, KJV)
Here we read 'The third day...'. Which third day is referred to here?
'The third day' can only be counted from the day of John's declaration of verses 35,36.
Again, we see that there could not have been any miracle between the baptism of Jesus through forty days in the wilderness till he called few disciples.
Jesus had not formed his 12 disciples at the time of the wedding in Cana of Galilee.
According to this chronology, the miracle of Lk.5:1-11 took place later than that of the wedding.
I hope this shows that the miracle at the wedding of Cana of Galilee was indeed the first miracle.
The timing/days could not afford that there was any miracle before this wedding.