Hi,
If you have installed the ros-electric-desktop-full then gazebo comes along with it.
You just have to follow the instructions here and get started.
If you have installed ros by other means and don't get gazebo along with it. Then checkout the stack from as given in the link controlled by svn and compile the package and follow the instructions in the first link of this answer.
Let me know if any issues.
Hope it helps,
Karthik
As you are not sure about your installation you may face dependencies issues later on. I suggest you to reinstall ROS desktop full version and get the Gazebo. Else you have to download the stack of Gazebo from the subversion using
svn co https://code.ros.org/svn/ros-pkg/stacks/simulator_gazebo/trunk/gazebo ~/ros_workspace/gazebo
followed by
cd ~/ros_workspace
rospack profile
rosmake gazebo
If you get any errors mostly would be for the dependencies which you may have to install individually till you get the gazebo package compiled successfully.
Hope it helps.
Karthik
edited my answer