The main theory that Jesus was trained as a carpenter is based on a verse in the Gospel according to Matthew, 13:55, with a parallel in Mark 6:3. The Matthew text refers to Jesus as "the carpenter's son", while the text in Mark refers to him as "the carpenter". The greek word is τέτκων (which transliterates as tektōn), which means a craftsman, and is generally taken to refer to a craftsman who worked with wood, possibly also general construction, the equivalent of a builder in some circumstances. It would be likely that if this were Joseph's trade, Jesus also learnt it.

All of which depends on the gospels being reliable sources, but that's were the idea comes from, which is the best answer to your question that you are likely to get.