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I would suggest going through the Creating a joint controller tutorial. Once you create a proper joint controller you will be able to subscribe to a state topic and get all information about the joint position, speed, load, etc. in proper ROS units (e.g. position in radians, speed in radians per second) instead of raw servo encoder ticks. Although you can subscribe to motor_states/ttyUSB0 topic, it really wasn't meant to be used directly. A joint controller will also provide a bunch of services through which you will be able to set joint speed, enable/disable torque, change torque limits, change compliance margin and slope among other things. There will also be a command topic which can be used to control you joint (sending a simple float value will move the motor).