It definitely depends on what type of app development you want to do and where you are starting from, but since my wheel house is web development I will answer

It definitely depends on what type of app development you want to do and where you are starting from, but since my wheel house is web development I will answer from here. Follow Brad Traversy on YouTube (TraversyMedia). Start with his "Crash course.." videos. Follow along a couple small project tutorials. Spend the money on his Udemy courses. But as others have said: Build stuff. Brad will walk you through the basics and then have you building things. FOllow along. Code with him. Depending on how much time you have to devote and grind, you could be further along in building web apps in 3 months than I was in 10 years.