You can add Card Fields to your List by hitting this button top right

Then selecting Manage Columns 


From here you can add, delete and edit existing Columns.


Press the + To create a new Column and Choose Field 


Then scroll through your Field options until you find the Field you want, this will show all available Fields in the Project, so some may not be on the Cards you are adding.  If you haven't created a Field yet, you'll need to,  for more on Fields go here. 




Hit Save & Close when done. 



You will now be able to edit the Field from the List, as well as the Card.