Having problems with using REST API and Postman

I am trying to use the REST API to pull down data from a GET process. I simply want to retrieve my data.

When I attempt to do it, all I get is a 403 - You don't have permission.

I have validated that I do have permission, so there is an issue some place else.

I am attempting to use Postman just to establish a basic process before I do any actual coding. I want to understand the processes, the steps involved, make sure my basic query is correct, etc. Everything I do just fails.

I've read the article on the Advanced Authentication with Postman. That won't help my situation because I won't be using Postman for my programs.