DC Motor with Rosserial help

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I am trying to make a controllable arduino robot. Is there anyone out there who knows how to make a motor turn without a motor shield (Is a motor shield really necessary)? Also, I am getting encoders for the DFRobot 4WD robot

DFRobot 4WD Arduino-Compatible Platform w / Encoders

and wanted to write a ROS driver for it. I have an xbee radio I'd like to put to use with it too. I have never written a driver, so, for those with experience, help would be great! And tips, tutorials, or general advice welcome!

thanks,

-Hunter A.