Is Oracle free to download?
If you're really only looking to learn about the development side and have no interest in the administration or installation side at the moment, a quicker route might be to download a prebuilt developer VM image for Virtualbox. That can get you up and running very quickly, and you can connect to the DB running inside the VM from outside, so you can continue to do your development in your current environment.

The overview echos what others have said about licensing kicking in once you ship an application (but Oracle licensing is a quagmire, you'll need to talk to Oracle about it if/when you get past playing around):


  Learning your way around a new software stack is challenging enough
  without having to spend multiple cycles on the install process.
  Instead, we have packaged such stacks into pre-built Oracle VM
  VirtualBox appliances that you can download, install, and experience
  as a single unit. Just downloaded/assemble the files, import into
  VirtualBox (available for free), import, and go (but not for
  production use or redistribution)!


For experimenting it's ideal, in my experience. Of course, it's unsupported, but as you won't have a support contract anyway that's a bit of a moot point.

As David said, PL/SQL is integral to the database, not a separate component, so it is available in the VM image. You get some tools too, including SQL Developer, but you might have less friction running that natively and connecting it to the DB in the VM.