So, for instance I have catalog called project in databricks free edition. If I would like to assign proper permission for my Service Principal (so that it can see the tables wihtin catalog and can query them) first I need to set 2 preequisite permission (red number 3 on screen below):

Here you can also specify SELECT permission in READ section - then all schemas and tables will be available for your service principal through inheritance.
If you want to grant your service principal access to selected table then pick that table and grant select permission on the table level.