What one command did God give Adam?

The first command given to man by God appears at Genesis 1:28 - "Be fruitful and become many and fill the earth and subdue it, and have in subjection the fish of the sea and the flying creatures of the heavens and every living creature that is moving upon the earth." (New World Translation)

This command, as Worded in this verse, was not given to Adam by name ("And God proceeded to create the man in his image, in God's image he created him; male and female he created them." - (Gen. 1:27)) However, this command was given before the end of His six creative "days", after which he declared that everything was good, so it is reasonable to infer that he had given this command to Adam and Eve before they sinned. They were the only humans living that that time.

However, there are two creation accounts in Genesis, merely told in a different narrative. Genesis chapter two does not repeat the command to "Be fruitful and become many and fill the earth". The first job God gave Adam in Genesis 2 was to name the animals. (Gen. 2:19)

Another perspective:Unless you see the command to "be fruitful" as a "job," Adam's first job was "Gardener of Eden."

Genesis 2:15 - Then the Lord God took the man and put him in the garden of Eden to tend and keep it.[NKJV]