SQL & PL/SQL

Ive just installed Oracle Express to help me go through a phase test. Ive created 3 tables under the SYSTEM username that I started the database with. Ive populated the tables with some data

Now whenever I try and enter any SELECT command it tells me

ORA-00942: table or view does not exist

Now a quick look on google tells me this is most likley that I don't have permission...however, I can't see what I can do to change that, I am the only user. If I create a new admin and recreate the tables it tells me the same.

Also, infuriatingly, it tells me that the 'show tables' is not a valid SQL command so I cannot make sure I am typing it correctly. Although obviously I have gone back into the object browser to check countless times.

Please help me out! Im assuming its something small, but google is huge and I don't even know where to begin with this.

Thanks
Mike