To add a new field to the form go to the Items -> Data Source Fields, drag and drop selected field to the form (1). You can add

Note! Before adding the field you need to add a Section. See more info for working with sections at Section Component article.

To add a field from related object in the "Selected field source" pick the related object (2) you need and the list of available fields will be replaced with related object fields. Drag and drop the field to the Form Section.

Each type of field has its icon (3):

The field has standard list of properties (see details at Components Basic Properties) and its own list of properties. Click the field on canvas (4) to modify field settings (5).

Validation rule allows to validate the field value before saving a record or by executing Validate form action.

Click "Add New" (6) at Validation Rules Section: 

Fill in Error Message, Validation Rules. 

You can also add Conditions and the Validation will work only when the Condition is true. For more information see proper article.

You can hide/show/disable/make required field conditionally. See more info at Working with Conditional Styles 

You can add onChange Events for the field. See more info on working with Events at Events article.