Make an app!
You could ignore everything else in this chapter if you just put this one thing into practice:
Don't wait until you are an expert before you start making apps!
Start building your dream app, the one with all the features, that's going to take Google Play by storm. A simple piece of advice, however, is this: do some planning first! But not too much; then get started.
Have some smaller and more easily achievable projects on the sidelines: projects you will be able to show to friends and family and that explore areas of Android that are new to you. If you are confident in these apps, you could upload them on Google Play. If you are worried about how they might be received by reviewers, then make them free and put a note in the description about it being "just a prototype" or something similar.
If your experience is anything like mine, you will find that as you read, study, and build apps, you will discover that your dream app can be improved...