The first Europeans to practice slavery in the US were the Spanish.

The first and oldest European settlement in the US is San Agustín, Florida. Founded in 1565 by the Spanish conquistador Pedro Menéndez de Avilés.

Slavery did not arise independently, it has been a thing as long as humanity itself (being practiced in one way or another).

Historical background: The Catholic monarchs of Spain made the newfound native Americans subjects of the Royal and Catholic Spanish Crown. They were 'enslaved' through a process called encomienda in which they were obligated to work.

Being enslaved in the proper sense did not happen to the native Americans (at least under Spanish control). They were 'enslaved' through the encomienda system and taxes in gold or other commodities.

Because the natives were weak and did not work, they resorted to importing African slaves. Also the African slaves did not have the status of the native Americans.

Where did they get this idea? The Europeans got this idea of relying on African slavery through the Arab slave trade.