The absolute best way to learn Photoshop for free is still through Photoshop tutorials. Practice these tutorials, use and create Photoshop resources and you will likely become a master of Adobe Photoshop.
The website I like most for Photoshop tutorials is PSDDude.
Free Photoshop Tutorials
For practicing Photoshop tutorials you'll probably find these useful too:
Free Photoshop Resources
and also useful
Free Photoshop Actions