Note: There is a good article written by Matthias Leibmann, Program Manager in the Exchange team, on using OAuth to access Calendar, Contact and Mail API in Office 365.
It's not as easy as doing a "GET" request to a single endpoint. You first have to go through the Oauth flow, of course, because you want to get meaningful user data.
I'm using Advanced REST client, an app for Chrome that you can obtain in the Chrome Web Store.
It seems that lots of folks use Fiddler, but I find it overly complicated and too heavy for this purpose. Advanced REST client is really lightweight and focuses on making REST requests and seeing responses. If you want more, check out Fiddler.
1. Assuming you have already registered your application with Azure (a post on how to do this will be coming up soon), you can use Advanced REST Client to make a GET authorization request:
In the header of the request, put: