I assume he was a carpenter by trade like Joseph. The bible doesn’t have a lot of detail on the years between his birth and events leading to the crucifixion but he probably did what all boys did and learned the trade of his father (Do we consider Joseph to be his father?)
We also know he did some fishing of both fish and men.
And Paul was a tent maker. He said he could have survived on contributions of the faithfull but he earned his living to be an example.