The majority of the slaves that were brought over to North America were used to work in the fields of plantations. Their jobs were primarily manual labor, such as picking tobacco, cotton, or working the fields. Some of the slaves were used as servants and for other tasks, but the majority of the people who became slaves brought over to North America were used for manual labor.
When slaves were sold at auction, they were looked at for their physical traits, so that slaveowners would buy the ones who were in the best shape to do the manual labor.