Getting started with ABCS is very easy, go to cloud.oracle.com, your starting point for all Oracle Cloud products. For every Oracle Cloud product applies that you can get a free trial through there. This trial is a great starting point for getting to know the product. If you go to the Platform section you will see Application Builder in the list. Once you went to ABCS specific site, you can register for a free trial.
It will take your email adres, phone number and some other info, after which you get a confirmation screen.
In this example I choose to create a web application, which launches a wizard form, you name your application and your choose a template.
After that you can start building your menu structure, do not worry, you can modify this again later, but when you have a idea for a structure you can supply it here:
Closing the wizard will take you to the quick tour that explains the basics of ABCS: