If neccesary:
sudo apt-get update
sudo apt-get upgrade
Then:
cd ~/catkin_ws/src/
git clone https://github.com/ROBOTIS-GIT/turtlebot3_msgs.git -b noetic-devel
git clone https://github.com/ROBOTIS-GIT/turtlebot3.git -b noetic-devel
git clone https://github.com/ROBOTIS-GIT/turtlebot3_simulations.git -b noetic-devel
cd ~/catkin_ws && catkin_make
source devel/setup.bash
Next:
export TURTLEBOT3_MODEL=waffle
or
export TURTLEBOT3_MODEL=burger
Finally pick one of these:
roslaunch turtlebot3_gazebo turtlebot3_empty_world.launch
roslaunch turtlebot3_gazebo turtlebot3_world.launch
roslaunch turtlebot3_gazebo turtlebot3_house.launch