NPM permissions error after package install

I'm a new user to Node and npm, working on Mac OSX 10.7.5. Here's what I did:

1. Installed the node-v0.10.31.pkg from the Node website.
2. Everything worked. The Node REPL works fine.
3. npm does not work. If I type $ npm install learnyounode -g, I get this error: -bash: npm: command not found

I understand npm is supposed to ship with node, so I don't understand why this is an issue. Browsing through some other responses to diagnose the problem...

$ /usr/local/bin/npm results in this message: -bash: /usr/local/bin/npm: Permission denied

$ which npm does not produce any result.

I've seen some say to use this command: curl http://npmjs.org/install.sh | sudo sh. But, I'm worried about installing over an existing install, and making things way way worse.

Can someone walk me through what my next step should be? Note that I'm not familiar with things like Homebrew, which I've also seen as a potential solution...

Thanks in advance. I just don't want to screw up my computer.