May be it will be better to make nobody the owner of the tables?
It's definitely a good idea to make somebody
else the owner of the table rather than SYSTEM... you shouldn't really be putting your own objects in the SYSTEM schema anyway if you can help it.
Whether NOBODY is the best schema to use depends on what you want the table for. If this is just a test system you are playing about with then it doesn't really matter... but if you are trying to put together a proper development/test system with a plan to move into a production environment then think through what you want (and whether NOBODY is the best name for your application schema).
Saying that, if this is just a database you are 'playing' with to learn some Oracle then you might want to look at granting system privileges rather than object privileges.
GRANT SELECT ANY TABLE TO ALEX;
Will let ALEX select form any table (other than the data dictionary).
Again, though, a word of caution - if you intend to put this into a 'real' system (rather than just one you are learning on) then think it through and only grant what you need. If you go for the 'easy' system privilege now you'll find you'll always be stuck with it because you'll never be 100% certain that you can revoke it without affecting your application.