If this is the first time running the development environment run yarn from the terminal to install the dependencies. If you do not have this command, see the docs for installation instructions. If you do not have node install, you should install it.
Next you are ready to run the development environment. This will start up a development server which you can use to navigate to the application. Keep this process running and builds will be triggered whenever you make a change.
sh npx netlify dev
Point yourself to the page, eg http://localhost:8888/
And you're off
If you need to do database stuff, make sure you have mongodb installed and its running (eg run mongod in the terminal). It can also help connect to mongo compass.