'A DC Motor' doesn't tell us much aprt from that its DC. This could be anything from the motor that makes my mobile phone vibrate to a 1000HP motor used in the steel mill. Knowing a bit more about the motor you're speaking of will allow us to give you a better answer.

The critical pieces of information are voltage and current. Lets assume we're talking about a model motor of around 12volts and less than three amperes. You could use a device like the L298 (see www.st.com) to interface your motor to the 8051. If your motor is smaller, something like a L293 might be a good choice.
These devices are called 'H' bridges due to the circuit configuration. There are lots of examples on the web.