Swift has made it so easy to get started in iOS.
Apple docs are 100%
If you're looking for tutorials - raywenderlich.com
They have great beginning iOS and Android tutorials. Also paid training courses too if that's something you're interested in.
Feel free to reach out too. The SO & Twitter communities are great because mobile devs are incredibly hard to find.
The Stanford Swift & iOS class is a wonderful resource, especially with transition from version 3 to 4 promising to be less source breaking than the previous 1-2 and 2-3 updates. Available in iTunes U, you get video lectures, theory, and practical exercises.
Lynda.com has many courses and sample code. I am concurrently watching Swift Essentials and other courses to better understand the big picture of the language while following the Stanford course.
Also look at the free Apple Swift guides on iBooks for fundamentals.
Most of my experience is in web development, but these days web technology and best practices have infiltrated mobile development in ways that make it a totally solid route to go down.
For Android, Google/Udacity have prepared the best online courses.
I won't recommend a total novice to go into react native. Past the tutorial part the learning curve is huge and not having strong experience on at least native development or web react will bring a lot of frustration.
Learning java is not essential btw, it's better to start with the framework and app structure, layouts, etc. And learn java deeply later.
Besides the one mentioned, you can try taking a look at the Stack Overflow Documentation section (stackoverflow.com/documentation), where for a certain platform there are introduction topics, like this one for iOS: stackoverflow.com/documentation/io... (there's a similar one for Android).
As for Learning Portals, I've tried teamtreehouse.com/ to get started from zero, and it was very clear. I think you have to pay, but I remember I used the trial time to follow the iOS and that was enough to get me started. (Tip: When I get impatient with the videos I put the speed to 1.25)
The free iOS Stanford course is fantastic: itunes.apple.com/gb/course/develop...
I enjoyed Flatiron School's free online Swift course: flatironschool.com/programs/online...
I found it fairly up-to-date (Swift 3) and surprisingly thorough.
Check out fusetools.com/
It's really easy to get apps running on mobile with it.
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