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This is more a Gazebo question than a ROS question (so I should refer you to the gazebosim.org site), but what you ask is described in one of the Gazebo tutorials, namely the Connect to ROS one.

If you've never done any of this (urdf, sdf, ROS, Gazebo, etc), I really recommend doing the ROS or (actually: and) Gazebo tutorials first (or find a book about it), and only then move on to more advanced things. Everything will seem really convoluted and incomprehensible otherwise (at least, that is what many students/people tell me is their experience).