以下の順にコマンドを打てばokです。
① brew install nodebrew
② mkdir -p ~/.nodebrew/src
③ nodebrew install-binary latest
④ nodebrew list
⑤ nodebrew use v11.12.0
⑥ echo 'export PATH=$HOME/.nodebrew/current/bin:$PATH' >> ~/.bash_profile
⑦ source ~/.bash_profile
⑧ npm -v