How do you get started in mobile application development?

I’m looking to redirect my career path to mobile app development. I transistioned to Systems Engineering from Software Development for a number of years. After being out of the development game for some time, is like a lifetime, how do I retool? Opportunties for mobile application development are abundant and I want to approach it with a strong level of competency. Unsure of where and the best approach to get started down the mobile development path…