Kinda new to this great program, I wonder if it is possible to edit fields shown in a portal. Or can I only edit the fields in the original database?
If so, how do I set the focus to that other database?
You absolutely can -- in fact you can use the portal to create new records if you like and prevent the user from ever using the related file directly.
To allow/diallow editing of fields simple toggle the "Allow Entry into Field" option under "Field Format'. If you choose to allow entry here then users can edit at will.
To allow/disallow creation of new records from the portal, select the "Allow creation of related records" toggle in the "Edit Relationship' window. Doing this will add a blank portal row at the bottom of the related records. When the user types data into these fields the "key" (field used to link in the related file) is automatically filled with an appropriate value.