Hey, I suppose that you already googled and try some answers on SO about that. I have a good solution how to fix the error with nodemon, when you do npm install -g and then nodemon server.js and see: Command not found.
I solved it with the next:
sudo npm install nodemon -g
sudo ln -s /usr/local/lib/node_modules/nodemon/bin/nodemon.js /usr/local/bin/nodemon
That's it, with this command we install nodemon, and then create a symbolic link. It works like a charm.