You can download 11GR2 version which is latest.

When ever the supports ends for one version it is always recommended to upgrade to the latest version. However, you always need to apply one-off / interim / CPU patches to the databases to keep them secure.

I suggest don't go into deep. There is a good chance to get confused. Start with concepts then move forward slowly.

SQL is structured query language which you might eventually learn while learning and working on Oracle database. PL/SQL is a programming language which you need to learn separately. In real world very few DBAs does PL/SQL programming.

Thanks,