I got stuck at this step: Preview the theme with example content

my root folder for creating the Hugo website is store in this folder: mysite. 

I then copied and pasted package.json and package-lock.json to "mysite" folder and typed "npm install". However, I got this error message: "zsh: command not found: npm". 

Could someone please help me with this issue... T^T 

Sorry if my question is tooo simple...I am still new to this...