Awesome that you want to learn something new :) Search hashnode for similar questions, they will help you understand the differences between using javascript and using Objective-C/Java for app building.

tl;dr is using native langs or something like React native depends on how complex you want the app to be. This is because native langs don't have the same limitations that React native does (and similar frameworks for building apps with javascript).

iOS requires you to buy a license to start developing (or at least last time I checked) so I would definitely start with android or react as they are both free to begin.

Personally, I like android/Java+XML but then again it's been over a year since I developed on it and I never developed with iOS (something that might change by the end of the year lol).

Wish you the best :)

Edit:
The community is strong regardless of which you choose.

As hipkiss mentioned in his answer... it is about the complexity of your application.. If you want to be able to develop complex and professional apps ... you have to go native.. meaning java for android ...or objective C /swift for ios React native is not a powerful tool for mobile applications... it cannot do but simple apps that doesn't contain much logic... if you want to see a sample ... search the play store for apps made by react or ionic and compare them with other apps... you will notice the difference by yourself. The only advantage about react or ionic is that they are multi platform ... you develop on android and the app can work also on ios ... but what is the use of that if the app is not powerful enough? Choosing android or ios ... this depends on many factors. .. one of them is your personal interest. .. other is the market need... in my country there is currently a growing need for ios developers, while the need for android is still big but not growing...I cannot tell about other places... Android as an operating system is making huge steps and the demand for its applications will never go away... and i believe its the same case for ios... its like the demand for Hyundai and Mercedes ... both have their types of customers.. Btw Android also has fees to make a developer account...25$ So good luck and welcome to the community :)