I wish this helpful for you Yes, before running, all dependencies must be installed. So you must run npm install. When developing, you can use npm install --save to install a dependency and automatically add it to package.json.
it helps some times As you said you are a beginner you can take the best independent modules for your project and start using. Instead of having a pre-defined package of necessary components as a single module. Problem is you may end up not understanding the whole pack and become more biased. So, I would suggest to use Expressjs (Best Choice) for url routing as mentioned above.
I wish this help you Globally installed modules are placed in "/usr/local/lib/node_modules" (or its equivalent on your system, which you can find by running "npm list -g"). As for the binaries that are globally installed, symbolic links to them are placed in "/usr/local/bin", but you can find out where they are on your system by using "npm bin -g".