use the XE editon, there is no 12c. There are some limits, but for developers at the beginning no real restriction.
The database software which is available for download over the OTN is under the development license and certainly can be used for learning purpose. I won't suggest to use XE edition if you are willing to work as a DBA because for production shops, its the Enterprise Edition (or SE) which is used everywhere.
That's the one I'm using! I don't know where the limits kick in though. I guess I haven't reached them yet.
Did you pay heed to the license agreement you accepted before downloading the software ? The usage restrictions are spelled out clearly.
The limits for 11g XE are also documented -