Whether you prefer an engine or directly using APIs, there's a solution for you to discover and try when building AR, VR or MR apps! However, before we dive into platforms, let's talk languages.
Unity is a real-time development platform that's great for creating 2D, 3D, AR & VR experiences. Like learning any new technology platform, the learning curve can be relatively steep if you've never used the platform for development. Fortunately, Unity provides a Learn platform that's pretty helpful for getting familiar with the interface as well as Unity Scripting. You can also choose to focus on specific areas of Unity development if you're already familiar with the platform. My advice would be to start with learning how to create for 2D first and then transition to 3D. C# knowledge comes in handy here given that you'll use C# for Unity scripting.
Alas, Unity isn't the only engine available. There's also Unreal.
Unreal is real-time engine for 3D development. More often, I find that XR developers prefer one platform over the other. Unreal provides a variety of free online learning courses for getting started with the platform. Their written documentation in the Unreal Editor Manual is also pretty extensive as well for getting started. C++ comes in handy here given that you'll use C++ for Unreal scripting.
Although I prefer to stick with Unity, I can honestly say that I've at least given Unreal try. However, I believe I'm biased because it took me a while to get used to Unity and now I couldn't imagine starting over. You may certainly have a different experience so definitely consider an intro course for both engines before settling on one.
But if working with a game engine and getting to know the UI isn't your cup of tea, there's always the web!
If you aren't quite sure as to where to get started, Microsoft provides some guidance on choosing a technology path as it relates to Mixed Reality.
Curious to learn more about realities? Follow the #MixedReality tag!