To create a table, enter it's name and save. If you're going to create multiple tables for your App, it will be helpful to give your tables meaningful names. Tables will be shared by all your Apps.
The first field you will be adding to your blank table is the Primary Key field (PK). Click "+ Add Primary Key Field" button to add your Primary Key field.
See how to add table fields Adding Table Fields
In the design phase, you can change your table structure by deleting the field and re-create it with the correct properties. Click Create Table button to write your table to your database.
As long as your table is empty, you can delete from the database and re-create anytime.
Once your table contains data, you won't be able to delete or change it.