Postman is an Open source platform to work with APIs. You can download postman from this link for free.
Click Download the App and select according to your Windows configuration.
Postman starts installing..
Postman opens and ask for Sign In, Enter your email details in next screen to sign in to postman.
Postman desktop version installed and opens with its launchpad.
You can also try web version of postman by clicking "Try the web version" button in downloads page and entering your email and personal details.
web version looks like this.
Congratulations!!! you have installed your postman successfully.