Bruno is a opensource, cross-platform app that makes it easy to experiment with APIs in a friendly interface. ShippingEasy's Bruno collection will help you get started quickly.
It is recommended to use one of the official API client libraries instead of making API calls yourself, but the Bruno collection includes a pre-request script that handles the request signing and can be used as an example of how the signing is done.
You can download Bruno here. It's available for Mac, Windows, and Linux.
The ShippingEasy API Example Repository is available on Github and contains an API library that you experiment with.