Where to start? To develop in mobile?

Answer 1

java and xml are used for android development.Start learning java here and go to udacity.com to learn android development.

Answer 2

Kotlin/Java with XMLis used for Android Apps Development.
Learn Java on SoloLearn. And move towards advanced courses as there's just basics on SoloLearn. Learn Android Development in Android Studio.

https://www.sololearn.com/discuss/580291/?ref=app
https://www.sololearn.com/discuss/823823/?ref=app
https://www.sololearn.com/discuss/823843/?ref=app