Great beginner resource. MIT App Inventor 2.
MIT appinventor 2: This site takes you through all you require to start creating apps right away. All you will need is a google email account. The tutorials are great taking you from very basic to quite complex ideas that you can build on.
This is an ideal site to whet your appetite.
A complimentary site that takes the MIT appinventor product and extends it new heights and gives you great tools and examples is the blog of Hossein Amerkashi. This turns the MIT appinventor into a much more useful tool and introduces lots of features and the examples are great.
Google Android Studio
The Android Studio development environment is more complex but gives you the full features of the android operating system. It takes longer to learn and is much more difficult than MIT App Inventor but having said that, this is the grown up product that gives you access to all the android features as new releases are made available.
Again there are great resources but these quickly become out of date as the new versions of android are released. That said any time you spend learning how to use this product is good and as you become familiar with it you start to understand what is going on and what you need to do if the tutorial or example you are working with does not seem to do what you expected.
Again this is a free product to download there is full documentation on what you need to do, what resources as far as your PC is required and any dependencies that the product requires.