To start your session run
$ foundry go in your terminal.
Once you execute the command, Foundry tries to find the
foundry.yaml configuration file in the same directory, connects you to your cloud development environment, and starts an interactive prompt with REPL for your Cloud Functions.
$ foundry init to generate the default
foundry.yaml configuration file. The configuration file must placed in the same directory as is the
package.json file for your Firebase Cloud Functions.
Learn more about the configuration file