It really depends on what you're aiming to do, and which programming language you know, but don't think you can learn from scratch and master a mobile development technology in days/month.
As I'm working in mobile development, I can say that you're left with 2, maybe 3 options :
As @Khaikaa said, Xamarin Forms might suit your needs. You work with C#, and can create one app for about every mobile plateform (yes, even windows phones) by "coding once".
If you know a bit of HTML + CSS, then Cordova might be a good option for you. I haven't worked with it much, but it seems pretty simple to use.
The last option is Flutter. It's pretty new, use Dart as programming language, but the development process is very different of what we are used to. So, pick that up only if you're not pleased by Xamarin Forms or Cordova.
I highly encourage you not to pick react-native: there is not enought components, too much troubles and hassle!