What should I learn first: react, angular or ASP.NET?

By the time you learn React, the UI community will have embraced something newr and coolr. Seriously, if I could advice a n00b developer, I would suggest they learn javascript very well, CSS, HTML, UNIX shell scripting, SQL, and pick one of : {Java, C#, Python}. With those skills as a foundation you can then specialize in a flavor of the month framework.

ASP.NET is primarily used for corporate IT type applications, usually internal facing ones. If that's the type of work you are doing or want to do, then knowing it is a good starting point. If you plan to work for "cool" tech companies, then a Linux based stack will be a better choice.

Of course, React and Angular both can serve as a front end to a .NET backend. In this area, Angular has been a bit more popular over the past few years but React is rapidly catching up since the learning curve is shorter. Right now, many companies are having trouble hiring people who know these frameworks in a .NET context.

One thing the consider with .NET in the corporate world is that there is often a considerable variation in how applications are built, depending on how long the development organization been in operation. It is common to see situations where they have everything from VB.NET Winforms apps originally written in 2003-2005 to Silverlight apps written in 2011 to brand new MVC/Angular apps.

I think React is a thinner layer of abstractions over plain JavaScript than Angular, so it should be easier to get started with.

So if these are your options, go for React.

If you got some jobs or projects in sight that need Angular, then I would go for Angular.

What's your background, what kinds of languages and environments do you currently work in?

Depends on the job you want.

What are you interested in making?