I would advise you to get a book. I can recommend you Murach's Oracle SQL and PL/SQL. Along with that you will get a free file to install a schema and all the related tables for practice. Explanations are pretty good and its worth all the money.
Get a book, and read up. The 4 basic queries -- understand the fundamentals how to query and transact data. P-SQL you can take up later. It is the basics which have to be strong.