To be clear with the following contents, let me tell you that the whole experiment or learning was done in a Windows environment, but then I found that MAC is easier to start with NodeJS.
Installation is straightforward. Just download the binary and get going.
Once installation is done, node and npm command should be available from command-line, if not try restarting and then test the commands. Testing the commands means typing “node” or “npm” in the command line and pressing ENTER. If installed, the result should be something other than “missing command or error message”.

The next commands I went through are

 npm root
npm bin 

why ? To see the details of below explained stuff

npm root // to see where modules go
npm bin // to see where executables go

[ ref : https://npmjs.org/doc/faq.html ]

Next I tried with “-g” switch as below.

 npm root -g
npm bin -g

Why ? Thats because node installs differently for global and local specifications.
[ ref : https://npmjs.org/doc/faq.html
If you install something with the -g flag, then its executables go in npm bin -g and its modules go in npm root -g ]

Next is my favorite first timer stuff, to list all the installed packages,

npm ls

Next to know a little bit about Node itself

 npm view npm author
npm view npm contributors

So thats the start.
Next is the node official doc. [ ref : http://nodeguide.com/beginner.html ]