As I can see in the screenshots, you have already created the sandbox cluster. In order to connect to the sandbox cluster from your mongo shell, please follow the below mentioned steps.
Open your atlas account, and then click on connect.
Once you click on connect a new prompt will appear on the screen. You need to click on Connect with the Mongo Shell.
After that you need to click on I have the Mongo Shell installed. Now at the bottom you can see a connection string. Go ahead and click on the Copy button.
Now you need to start your Command line and paste the connection string into it. You will be asked to enter the password. Enter the password that you created while setting up your Sandbox cluster.
Congratulations , now you are connected to your sandbox cluster.
If you have any other query, feel free to reach out to us. Happy Learning !
Curriculum Support Engineer