Below is a list of websites which have plenty of tutorials for App Inventor. Take a look, might help you.
If you prefer a good reference by your side, you might want to get the book titled App Inventor. Not only this book was written by the developers of App Inventor team, they also know how to explain things the simplest way possible.
This book covers a plenty of tutorials from beginners to intermediate with step by step instructions for you to follow. If you do all the tutorials provided in the book, you can move forward without any problem.