The easier option, is on the form itself that you open click on the Form Editor button. If you don't see the button, you can click on the ... (3 dots) at the right of all the command bar buttons and then click on Form Editor.
If that doesn't work, navigate to Settings -> Customizations -> Customize the System.
On the left hand side navigation, find the entity that you want to customize, and expand it.
Click on Forms and open the Main Form that you want to customize. The name of the form will usually be Information or the Name of the Entity.
In the main for you will have a view of what you see when you actually access the form.
You can then add/remove fields from the form.
For setting rules on particular actions, you can use either Business Rules or JavaScript.
You can learn how to create Business Rules by clicking here:
docs.microsoft.com/.../create-business-rules-recommendations-apply-logic-form
Hope this helps.