After the Flood, what did Noah build first according to Genesis?

After the Flood, Noah first built an altar according to Genesis.

Genesis 8:20 says that Noah built an altar to the Lord after leaving the ark. He then took some clean animals and clean birds and offered burnt offerings on it. The sequence occurs after the ground had dried and God instructed Noah, his family, and the animals to leave the vessel.

The altar is significant because it marks the first recorded act of worship in the post-Flood world. It also connects with the earlier distinction between clean and unclean animals: Genesis 7 gives Noah extra clean animals, and Genesis 8 provides a narrative context for that provision.

Noah did not first build a city or tower in the Genesis account. Later chapters describe his vineyard and family events, but the immediate construction named after his departure from the ark is an altar.