I want generate an new form in a custom module via the generate:form
command.
In this step, I must enter a field type.
How can I get all available field types?
With the term "field type" you probably don't find what you are looking for. Better search for "form and render elements". You can find a list of all available elements here:
https://api.drupal.org/api/drupal/elements/8.2.x
You can use all elements of the type "FormElement".