You first have to build an app that someone considers useful.
Then, search this forum, using the Search for messages block above using the keyword monetize.
This question has been asked many times before, so we want you to be aware that developers can get answers to question by using the Forum's search features.
We want you to have fun with App Inventor so here are some resources to help you learn to use the AI2 tools A very good way to learn App Inventor is to read the free Inventor's Manual here in the AI2 free online eBook http://www.appinventor.org/book2
... the links are at the bottom of the Web page. The book 'teaches' users how to program with AI2 blocks.