oi Gustavo, ta usando linux ou windows? Se foi no linux, depois que vc instalou, vc fechou e abriu o terminal?

windows...

npm WARN onload-script failed to require onload script npm-autoinit/autoinit
npm WARN onload-script Error: Cannot find module 'npm-autoinit/autoinit'
npm WARN onload-script     at Function.Module._resolveFilename (module.js:325:15)
npm WARN onload-script     at Function.Module._load (module.js:276:25)
npm WARN onload-script     at Module.require (module.js:353:17)
npm WARN onload-script     at require (internal/module.js:12:17)
npm WARN onload-script     at cb (C:\Program Files\nodejs\node_modules\npm\lib\npm.js:289:9)
npm WARN onload-script     at C:\Program Files\nodejs\node_modules\npm\lib\npm.js:382:14
npm WARN onload-script     at C:\Program Files\nodejs\node_modules\npm\lib\config\core.js:82:7
npm WARN onload-script     at Array.forEach (native)
npm WARN onload-script     at C:\Program Files\nodejs\node_modules\npm\lib\config\core.js:81:13
npm WARN onload-script     at f (C:\Program Files\nodejs\node_modules\npm\node_modules\once\once.js:17:25)
npm WARN onload-script  { [Error: Cannot find module 'npm-autoinit/autoinit'] code: 'MODULE_NOT_FOUND' }
npm WARN onload-script failed to require onload script npm-autoinit/autoinit
npm WARN onload-script Error: Cannot find module 'npm-autoinit/autoinit'
npm WARN onload-script     at Function.Module._resolveFilename (module.js:325:15)
npm WARN onload-script     at Function.Module._load (module.js:276:25)
npm WARN onload-script     at Module.require (module.js:353:17)
npm WARN onload-script     at require (internal/module.js:12:17)
npm WARN onload-script     at cb (C:\Program Files\nodejs\node_modules\npm\lib\npm.js:289:9)
npm WARN onload-script     at C:\Program Files\nodejs\node_modules\npm\lib\npm.js:382:14
npm WARN onload-script     at C:\Program Files\nodejs\node_modules\npm\lib\config\core.js:82:7
npm WARN onload-script     at Array.forEach (native)
npm WARN onload-script     at C:\Program Files\nodejs\node_modules\npm\lib\config\core.js:81:13
npm WARN onload-script     at f (C:\Program Files\nodejs\node_modules\npm\node_modules\once\once.js:17:25)
npm WARN onload-script  { [Error: Cannot find module 'npm-autoinit/autoinit'] code: 'MODULE_NOT_FOUND' }
npm WARN optional dep failed, continuing fsevents@1.0.14
C:\Users\Gustavo\npm\nodemon -> C:\Users\Gustavo\npm\node_modules\nodemon\bin\nodemon.js
nodemon@1.10.0 C:\Users\Gustavo\npm\node_modules\nodemon
├── ignore-by-default@1.0.1
├── undefsafe@0.0.3
├── es6-promise@3.2.1
├── debug@2.2.0 (ms@0.7.1)
├── minimatch@3.0.2 (brace-expansion@1.1.6)
├── touch@1.0.0 (nopt@1.0.10)
├── lodash.defaults@3.1.2 (lodash.restparam@3.6.1, lodash.assign@3.2.0)
├── ps-tree@1.1.0 (event-stream@3.3.4)
├── chokidar@1.6.0 (path-is-absolute@1.0.0, inherits@2.0.1, async-each@1.0.0, glob-parent@2.0.0, is-glob@2.0.1, is-binary-path@1.0.1, readdirp@2.1.0, anymatch@1.3.0)
└── update-notifier@0.5.0 (is-npm@1.0.0, semver-diff@2.1.0, string-length@1.0.1, chalk@1.1.3, repeating@1.1.3, configstore@1.4.0, latest-version@1.0.1)

C:\Users\Gustavo\Desktop\teste-node1>nodemon app
'nodemon' não é reconhecido como um comando interno
ou externo, um programa operável ou um arquivo em lotes.

esse é o erro

só para a gente fazer um teste... tenta fazer assim:

node C:\Users\Gustavo\npm\node_modules\nodemon\bin\nodemon.js

já resolvi o problema, obrigado!