Slavery existed long before Africa got into picture. Europeans enslaved each other before they enslaved Africans.
Slave trade started in the middle of the 15th century and last until the end of the 19th century.
The first European nation that began the slave trade was Portugal.
Other European Kindgdoms joined the trade too, it happened when they were able to expand overseas and reach Africa.
The first ones to be kidnapped were the people from the west coast of Africa. The kiddnapped Africans were brought to Europe and after the European discovered the American continent they were sent there too.
They were not seen or treated as human beings, they were transported in crowded ships with animals and many of them died along the way.