[Authentication, c#,Web API] need some help selecting a simple authentication system for asp.net Web API.

<div> Hi all, I'm in the process of building an application/solution. The individual parts now work in a proof of concept. So I'm now busy with knitting it all together in one demo-able solution. In the workflow i'm thinking of, i'm making use of users. So that means users logging in and authentication. I thought I wasn't the only one that had to solve this and there would be a lot of information about it online. And luckily I was right, but there are so many different ways to do this and a lot of documentation online only describes a part of the system not the complete authentication and authorizing system. So long story short, i'm being drowned in a sea of information. What i'm hoping for is that someone know a relatively simply solution that will fulfill my requirements.
What do I have, what do I need:
The desktop application is made with c# 
</div> <div>The online component I plan on making wit ...