Gen 8:20 Then Noah built an altar to the LORD and took some of every clean animal and some of every clean bird and offered burnt offerings on the altar.
-no, i'm not going to say that this is the first altar. the first ones mentioned were Cain and Abel's. but if i'm going to ask what is the first thing Noah did after getting off the ark...well, there's your answer! Noah really had his priorities set. there was a whole world waiting for him to explore after months cooped up in a dark ark but he put God first. i wish i can always be like that.
-a spoiler: you will notice that in every place that the people of God settle, they will always set up an altar first. well, almost always. in history, it also became a practice for a certain people (i forgot which) to always build a church first. even before they built their own houses. cool huh?
-interesting observation: God never commanded Noah to make an altar...but He did tell Noah to take 7 of each clean animal before boarding the ark, aside from the 2 of each kind of animal command.