you need to grant yourself access to those objects.
if you log on as sys you can do select * from scott.emp for example.
If you have an account without dba priveleges you need to grant access to them, so e.g. if you have account called john you would do grant select on scott.emp to john as either the sys user or scott user.
Then as john you would need to do select * from scott.emp;
By default in Oracle you have no access to anything apart from your own objects. If you want full access I suggest you create a role and grant select any table to that role (along with whatever other privs you need, e.g update and delete)
I also wouldnt recommend this in a production system - asking for trouble, however if its only for tesitng go for it
There are accounts called sys and system which will allow access to anything, but they are special accounts and they are not to be used for general day to day use. So my advice is to create you own user with the right privs you need - a good habit to get into.