To learn linux just start using it.
The easy way would be to install
Ubuntu, which is good all purpose operating system.
More difficult, but starting your learn process immediately, would be to install
Arch. As a bonus you get a cutting edge Linux. Arch has also a very comprehensive guides on most problems you can get yourself into.
If you seek more general knowledge, you should start with
Unix perhaps. Linux is just one of the many Unix flavors.