According to Genesis, Noah first built an altar after the floodwaters receded and he left the Ark.
Genesis 8:18–20 describes Noah, his family, and the animals leaving the ark. The next recorded action is Noah's construction of an altar to the Lord, followed by burnt offerings from clean animals and clean birds. This is the narrative's first post-flood act after disembarkation.
Only later does Genesis describe Noah as planting a vineyard. The rainbow appears in the covenant passage that follows the sacrifice, so it is not something Noah names or creates. These later details often become blended together in simplified versions of the story.
The altar also links the end of the flood with the covenant that follows. Genesis presents the sacrifice, God's promise concerning future floods, and the rainbow sign as a connected sequence. The answer is therefore a specific action recorded immediately after Noah leaves the vessel, rather than a general description of rebuilding life.