Joystick operation for a gazebo ros youbot

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As this tutorial shows, you need to subscribe to your topic /joy and then assign the data from the joystick (with a proper normalization/scaling) to a Twist message which you then can publish to your /cmd_vel topic in order to move your youBot.

The tutorial shows an example of the callback, i.e. where you listen to the joystick data and then assign it to the Twist message:

void TeleopTurtle::joyCallback(const sensor_msgs::Joy::ConstPtr& joy)
{
  geometry_msgs::Twist twist;
  twist.angular.z = a_scale_*joy->axes[angular_];
  twist.linear.x = l_scale_*joy->axes[linear_];
  vel_pub_.publish(twist);
}