You will have to clone this repo using the flag --recursive because it has other repositories as dependencies.
Clone it into your catkin workspace. Then, compile it. If there are some problems, maybe it will be fixed using catkin_make -j1.
This packages is located under the sim folder. It contains all you need to be able to launch a Gazebo simulation.
You can run it with the following command:
$ roslaunch tb3_sim tb3_sim_gui.launch # GUI
$ roslaunch tb3_sim tb3_sim_nogui.launch # Not GUI
NOTE: Under the basics folder you will find a lot of ROS code examples in Python. Next steps: Add C++ code.